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.