videoplayer: adapt lateness detection and dropping to buffering
video player: present correct pts to user for a/v sync (after buffering in renderer)
videoplayer: some rework and documentation
Revert "dvdplayer: disable buffering unit dropping is improves"

This reverts commit de1caf5686c1fb53cb7ab11b356e6c22770740db.
OMXPlayerVideo: adapt to change in FlipPage