demux_lavf: support new metadata update API

This Libav-invented API is of course completely different from the
FFmpeg-one. (The fun part is that I approved of both.)
vo: fix mingw compilation
vo: remove unused libavutil include
client API: adjust documentation for mpv_suspend()

It's not true anymore that the core will stop replying for 50ms
(waiting for video) without calling this function. Simplify the
documentation accordingly. Accessing properties that go through
the VO still have this problem, though.
lua: allow disabling suspend

I'd like to enable this by default, but unfortunately the OSC seems to
have some problems with it.
client API: directly lock playloop

Until recently, vo_opengl could be accessed from a single thread only,
due to the OpenGL API context being thread-specific. This issue doesn't
exist anymore, because VOs run on their own thread. This means we can
simply lock/unlock the playloop instead of doing something complicated
to get the playloop thread to execute our code.
sub: call sub_reset() on seeks only

sub_reset() was called on cycling subtitle tracks and on seeking. Since
we don't want that subtitles disppear on cycling, sd_lavc.c didn't clear
its internal subtitle queue on reset, which meant that seeking with PGS
subtitles could leave the subtitle on screen (PGS subtitles usually
don't have a duration set).

Call it only on seeking, so we can also strictly clear the subtitle
queue in sd_lavc.

(This still can go very wrong if you disable a subtitle, seek, and
enable it again - for example, if used with libavformat that uses "SSA"
style demuxed ASS subtitle packets. That shouldn't happen with newer
libavformat versions, and the user can "correct" it anyway by executing
a seek while the subtitle is selected.)
sub: add option to workaround broken mkv files

See additions to options.rst.
osc: Overhaul (wip)

Code reorganized to make layouts exchangeable
alternative test layout can be tested with
layout=slimbox
in the OSC config

timers are now used to properly animate the fade out when the
player is paused

duplicate seeks are discarded again
lua: wake up the core when setting OSD

The OSD is marked as changed, but the core isn't notified and this
doesn't immediately wakeup. (Possibly the OSD code should wakeup the
core instead, but maybe that woudl be overkill.)

Observed when using "mp.use_suspend = false" in the OSC, and pausing,
and moving the mouse pointer out of the window. The last part of the
fade remained visible for longer than intended.
player: use virtual time for --audio-file with ordered chapters

Apparently users prefer this behavior.

It was used for subtitles too, so move the code to calculate the video
offset into a separate function. Seeking also needs to be fixed.

Fixes #1018.
vo: eliminate a redundant variable

Originally, I probably had plans to allow NULL images to handle things
like the last frame case, but that idea was dropped later.
video: add VO framedropping mode

This mostly uses the same idea as with vo_vdpau.c, but much simplified.

On X11, it tries to get the display framerate with XF86VM, and limits
the frequency of new video frames against it. Note that this is an old
extension, and is confirmed not to work correctly with multi-monitor
setups. But we're using it because it was already around (it is also
used by vo_vdpau).

This attempts to predict the next vsync event by using the time of the
last frame and the display FPS. Even if that goes completely wrong,
the results are still relatively good.

On other systems, or if the X11 code doesn't return a display FPS, a
framerate of 1000 is assumed. This is infinite for all practical
purposes, and means that only frames which are definitely too late are
dropped. This probably has worse results, but is still useful.

"--framedrop=yes" is basically replaced with "--framedrop=decoder". The
old framedropping mode is kept around, and should perhaps be improved.
Dropping on the decoder level is still useful if decoding itself is too
slow.
vo_opengl: if glfinish is used, also call it after swappping

It seems that at least on nvidia systems with composting disabled, we
can get it to block deterministically on the actual vsync event, which
should improve framedropping.
gl_common: add SGI_video_sync extension
vo_opengl: optional support for using GLX_SGI_video_sync

I'm not sure about the merit, though it does print nice numbers if debug
output is enabled.

Basically, this tries to achieve similar results as the glFinish()
business, but again it entirely depends on the drivers whether this
does anything meaningful, or whether it's actively harmful.
audio: add a mode to insert silence on severe A/V desync

This is probably a stupid idea, but it can't be denied that this
actually allows playing video without larger desync, even if video is
too slow.
video: add --display-fps switch to control framedrop FPS

Since the display FPS is currently detected on X11 only (and even there
it's known to be wrong on certain setups), it seems like a good idea to
make this user-configurable.
vo_wayland: fix redrawing logic

I must have broken it some time ago. The error case dealing with an
unavailable backbuffer was broken, and didn't handle memory management
correctly.
player: cosmetics: make code more compact