sd_ass: add a very simple and evil way to override ASS subtitle styles

--ass-style-override=force now attempts to override the 'Default' style.
May or may not work. In some situations it will work, but also mess up
seemingly unrelated things like signs typeset with ASS.
vo_opengl: Include :icc-approx-gamma option in the 3DLUT cache header

This makes sure the ICC cache is recomputed when the :icc-approx-gamma
option is changed, since it affects the output quite a lot.
vo_opengl: minor simplification

I think this is easier to follow.
client API: fix playloop thread wakeup

The playloop usually waits in select(), using a timeout needed for
refilling audio and video buffers. This means the client API needs
a separate mechanism to interrupt the select() call. This mechanism
exists, but I forgot to use it. This commit fixes it.

If it works, this will make the client API react faster, epsecially
in audio-only mode. If video is enabled, the reaction time is capped
to 50ms (or somewhat faster if the framerate is >20 fps), because
the playloop stops reacting to anything in order to render and time
the next video frame. (This will be fixed later by moving the VO
to its own thread.)
Merge remote-tracking branch 'upstream/master'