ao_sdl: make sure our buffer is always larger than what SDL requests

Assume obtained.samples contains the number of samples the SDL audio
callback will request at once. Then make sure ao.c will set the buffer
size at least to 3 times that value (or more).

Might help with bad SDL audio backends like ESD, which supposedly uses a
500ms buffer.
vo_opengl: Use bt709_expand on OSD for :srgb

This affects the OSD only when :srgb is enabled, this still used the old
gamma approximation of 2.22 previously.
vo_opengl: Simplify and clarify color correction code

This commit:

- Changes some of the #define and variable names for clarification and
  adds comments where appropriate.
- Unifies :srgb and :icc-profile, making them fit into the same step of
  the decoding process and removing the weird interactions between both
  of them.
- Makes :icc-profile take precedence over :srgb (to significantly reduce
  the number of confusing and useless special cases)
- Moves BT709 decompanding (approximate or actual) to the shader in all
  cases, making it happen before upscaling (instead of the old 0.45
  gamma function). This is the simpler and more proper way to do it.
- Enables the approx gamma function to work with :srgb as well due to
  this (since they now share the gamma expansion code).
- Renames :icc-approx-gamma to :approx-gamma since it is no longer tied
  to the ICC options or LittleCMS.
- Uses gamma 2.4 as input space for the actual 3DLUT, this is now a
  pretty arbitrary factor but I picked 2.4 mainly because a higher pure
  power value here seems to produce visually better results with wide
  gamut profiles, rather then the previous 1.95 or BT.709.
- Adds the input gamma space to the 3dlut cache header in case we change
  it more in the future, or even make it user customizable (though I
  don't see why the latter would really be necessary).
- Fixes the OSD's gamma when using :srgb, which was previously still
  using the old (0.45) approximation in all cases.
- Updates documentation on :srgb, it was still mentioning the old
  behavior from circa a year ago.

This commit should serve to both open up and make the CMS/shader code much
more accessible and less confusing/error-prone and simultaneously also
improve the performance of 3DLUTs with wide gamut color spaces.

I would liked to have made it more modular but almost all of these
changes are interdependent, save for the documentation updates.

Note: Right now, the "3DLUT takes precedence over SRGB" logic is just
coded into gl_lcms.c's compile_shaders function. Ideally, this should be
done earlier, when parsing the options (by overriding the actual
opts.srgb flag) and output a warning to the user.

Note: I'm not sure how well this works together with real-world
subtitles that may need to be color corrected as well. I'm not sure
whether :approx-gamma needs to apply to subtitles as well. I'll need to
test this on proper files later.

Note: As of now, linear light scaling is still intrinsically tied to
either :srgb or :icc-profile. It would be thinkable to have this as an
extra option, :linear-scaling or similar, that could be used with or
without the two color management options.
vo_opengl: Correct and clarify gl_check_features

This updates the logic for the new, somewhat unified behavior of SRGB
and 3DLUT since 34bf9be (not that it was particularly correct even that
change) and checks for the presence of corresponding extensions only in
the cases in which they're needed.
video: initialize hw decoder in get_format

Apparently the "right" place to initialize the hardware decoder is in
the libavcodec get_format callback.

This doesn't change vda.c and vdpau_old.c, because I don't have OSX, and
vdpau_old.c is probably going to be removed soon (if Libav ever manages
to release Libav 10). So for now the init_decoder callback added with
this commit is optional.

This also means vdpau.c and vaapi.c don't have to manage and check the
image parameters anymore.

This change is probably needed for when libavcodec VDA supports gets a
new iteration of its API.
Revert "player: simplify audio reset when seeking"

This reverts commit 75dd3ec2106701cb865f52966de66c51cb6f9204.

This broke seeking with ordered chapters in some situations. While
the reverted commit was perfectly fine for playback of normal files,
it overlooked that in the ordered chapters case switching segments
actually reinitialized the audio chain completely, including the
decoder. And decoders still read packets on initialization. We can
restore the original commit as soon as decoders stop doing this.
options.rst: correct broken URL
libmpv: add pkg-config file
build: install client.h under mpv/ instead of libmpv/
ao_wasapi: Initial conversion to the new pull model

Gets rid of the internal ring buffer and get_buffer. Corrects an
implementation error in thread_reset.

There is still a possible race condition on reset, and a few refactors
left to do. If feasible, the thread that handles everything
WASAPI-related will be made to only handle feed events.
ao_wasapi: Split into 2 files

ao_wasapi.c was almost entirely init code mixed with option code and
occasionally actual audio handling code. Split most things to
ao_wasapi_utils.c and keep the audio handling code in ao_wasapi.c.
ao_wasapi: Move non-critical code outside of the event thread

Due to the COM Single-Threaded Apartment model, the thread owning the
objects will still do all the actual method calls (in the form of
message dispatches), but at least this will be COM's problem rather than
having to set up several handles and adding extra code to the event
thread.

Since the event thread still needs to own the WASAPI handles to avoid
waiting on another thread to dispatch the messages, the init and uninit
code still has to run in the thread.

This also removes a broken drain implementation and removes unused
headers from each of the files split from the original ao_wasapi.c.
ao_wasapi: Implement per-application mixing

The volume controls in mpv now affect the session's volume (the
application's volume in the mixer). Since we do not request a
non-persistent session, the volume and mute status persist across mpv
invocations and system reboots.

In exclusive mode, WASAPI doesn't have access to a mixer so the endpoint
(sound card)'s master volume is modified instead. Since by definition
mpv is the only thing outputting audio in exclusive mode, this causes no
conflict, and ao_wasapi restores the last user-set volume when it's
uninitialized.
ao_wasapi: Implement AOCONTROL_UPDATE_STREAM_TITLE
ao_wasapi: Use the character set conversion functions from io.h

...rather than rolling out our own. The only possible advantage is that
the "custom" ones didn't use talloc.
build: fix version in mpv.pc
build: rename --enable-shared switch

Rename it to --enable-libmpv-shared. The option name didn't really
tell much. When we add the possibility to create a static library,
it would also be bad if that were named --enable-static (because it
would sound like it does what --static-build does).
build: automagically extract client library version

This reads MPV_CLIENT_API_VERSION from the source header, and turns it
into a 3 part version number.

E.g. if MPV_CLIENT_API_VERSION were 0x12abcdef, this would result in
"18.171.773615" (8 bits, 8 bits, 16 bits).

We'll see if this is actually useful, or if it's too clever.
stream_file: network file system detection for Linux

Addresses issue #558 on Linux systems.

Signed-off-by: wm4 <wm4@nowhere>
build: make check for BSD fstatfs() a bit more strict

Linux also has fstatfs(), and the test relied on certain system include
files being available on BSD, but not on Linux. It would break if Linux
added the missing includes for some reason.

Make it a bit stricter, and check for the struct statfs field the code
needs.