[rbp/omxplayer] Remove flip timing logic from OMXPlayerVideo

Currently subs are ~500ms out of sync with a lot of jitter.

The video is not rendered by the arm, so a lot of the page flipping code is redundant.
Calling ProcessOverlays/FlipPage every frame is a big performance hit on Pi,
so I keep track of when the subs will next change and only call them then.

This has the side effect that ASS/SSA subs don't animate, so while sub is onscreen,
I periodically update (limited 10 fps).
[rbp/omxplayer] Remove discontinuity handling from players

I think sync/continuity checking done in OMXPlayerVideo and OMXPlayerAudio on Pi
is always wrong and leads to audio sync errors after discontinuities.

The GPU already does clock adjustment based on discontinuities in timestamps,
and is in a better position to do this adjustment.
(Remember there are a couple of seconds of buffered audio/video between here
and GPU and we want to adjust the clock at the other end of this queue).
Merge pull request #2853 from popcornmix/remove_discontinuity

[rbp/omxplayer] Remove discontinuity handling from players