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.