[rbp/omxplayer] Workaround for audio_mixer is not required

We have similar code in command line omxplayer, and that has never had this
workaround. I've been running with it removed for some weeks with no problems.

I believe it was working around a bug elsewhere that is no longer present.
[rbp/omxplayer] Add calls for stepping and retreiving the clock adjustment
[rbp/omxplayer] Use OMXPlaySpeed for pause/resume
[rbp/omxplayer] dispmanx now handles negative rectangles correctly so adjustment not required