[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 code up from OMXVideo to OMXPlayerVideo.
[rbp/omxplayer] More care with marking GetCurrentPts as invalid
Merge pull request #2925 from popcornmix/gpu_timestamps

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