[rbp] Set mediatime on GPU after a seek.

Currently after a seek, the mediatime doesn't update until an audio and video packet have been fetched from demuxer, and been decoded. For HD content this can take a second or two.
This has a couple of undesirable effects:
The seek time that pops up after a seek initially shows the before seek time, and the file progress bar is laggy.
If you seek a second time, before the time has updated, it will use the before seek time, and the second seek has no effect.
This limits a sequence of seeks to a maximum of about 1 every second or two.

The fix udpates the GPU mediatime immediately after the seek, so the mediatime is correct immediately which fixes the undesirable behaviour.