vd_lavc: reinit hwdec on profile changes

Needed in theory. I don't know if there are even any real-world files
which change the profile mid-stream.
img_format: AV_PIX_FMT_FLAG_ALPHA is always available

We no more support ancient libavutil versions.
video: change image format from unsigned int to int in some places

Image formats used to be FourCCs, so unsigned int was better. But now
it's annoying and the only difference is that unsigned int is more to
type than int.
vdpau: remove unused type
vdpau: remove legacy pixel formats

They were used by ancient libavcodec versions. This also removes the
need to distinguish vdpau image formats at all (since there is only
one), and some code can be simplified.
mp_image_pool: add features needed for use with hwaccel

The plan is to get rid of the custom VAAPI and possibly VDPAU surface
allocators.

Add custom surface allocation, because hwaccel surfaces are allocated
completely differently from software surfaces.

Add optional LRU allocation, which is (probably) helpful for hwaccel,
but (probably) less optimal for software surfaces.

mp_image_pool_get_no_alloc() is specifically for VAAPI, which can't
allocate new decoder surfaces after decoder init.
vaapi: replace image pool implementation with mp_image_pool

Although I at first thought it would be better to have a separate
implementation for hwaccels because the difference to software images
are too large, it turns out you can actually save some code with it.

Note that the old implementation had a small memory management bug. This
got painted over in commit 269c1e1, but is hereby solved properly.

Also note that I couldn't test vf_vavpp.c (due to lack of hardware), and
I hope I didn't accidentally break it.
vaapi: make struct va_surface private

It's not really needed to be public. Other code can just use mp_image.
The only disadvantage is that the other code needs to call an accessor
to get the VASurfaceID.
vaapi: simplify

Merge va_surface_priv into va_surface.
vaapi: solve potential problem with ambiguous image formats

VAAPI has some ambiguous image formats, like VA_FOURCC_I420,
VA_FOURCC_IYUV, VA_FOURCC_YV12 (the latter exactly the same as the first
two, just with swapped planes). There is potentially a problem when one
specific VAAPI format was picked, and converting it to a mpv format and
back to a VAAPI FourCC would result in a numerically different format
(even if it's actually the same). Then it could e.g. happen that
functions like va_surface_upload() reallocate the underlying VAImage,
which would be inefficient. Change the code so that this can't happen.

(Probably not a problem in practice with the current VAAPI usage.)
command, lua: change script_message semantics

Change script_message to broadcast the message to all clients. Add a new
script_message_to command, which does what the old script_message
command did.

This is intended as simplification, although it might lead to chaos too.
lua: rename mp.register_script_command() to mp.register_script_message()

More consistent naming.
osc: add enable/disable message, and map DEL to disabling the OSC

"enable-osc" will make the OSC appear at any time (although it'll
quickly disappear again if the mouse is not inside the OSC). "disable-
osc" will make it disappear permanently.

Also, if the OSC is visible, force remap the DEL key to make the OSC
disappear.
build: only check for Linux fstatfs on Linux

This check incorrectly passed on Cygwin. While Cygwin has the
statfs.f_type field, it contains the volume's FileSystemAttributes,
which aren't useful for detecting network mounts.
stream_dvd/stream_dvdnav: make disc-title for DVDs start from 0

This commit makes 'disc-title' properties for DVDs start from 0.
There was an inconsistency around 'disc-title' property between
DVDs (from 1) and Blu-rays (from 0). This fixes #648.

Signed-off-by: wm4 <wm4@nowhere>
options: fix allowed DVD start title
changes.rst: add dvd://1 -> dvd://0 change

Done two commits ago.