[rbp/omxplayer] Use timestamps given to gpu as GetCurrentPts

Currently the timestamps gpu is using and those returned from GetCurrentPts can diverge, making decisions inaccurate.
Make sure they use the same timestamps by bringing the timestamp adjustment code up from OMXVideo to OMXPlayerVideo.

This is required for a future commit that calculates time to underrun as GetCurrentPts-OMXMediaTime.