[rbp/omxplayer] Avoid too many calls to GPU

We currently read the media time once per packet received from demuxer to determine gpu underrun.
We've found that TrueHD audio in particular produces ~1000 packets per second (whether it is the active track or not).
The cost of reading media time (from gpu) is high enough that 1000 calls per second makes us fail to keep up.
So, cache the media time, and only read it at most 50 times per second.