[rbp] Fix for broken ASS subtitles.

The video fifo patch broke some types of subtitles including ASS.
This keeps closer track of when the sleep time would have ended and calls FlipPage at the appropriate time.
I've also removed the 500ms in the sleep time calculation as that makes the subs render 500ms late. Not sure what its purpose was.
Merge pull request #2206 from popcornmix/fix_video_fifo_ass

[rbp] Fix for broken ASS subtitles.