[rbp/omxplayer] Remove flush of video codec when audio codec changes

I believe this was added to work around a race condition that is no longer present.
It discards video and results in a corruption until the next I frame.
[rbp/omxplayer] Reduce the size of preroll

The preroll defines how much earlier the clock starts compares to timestamps of audio/video.
This needs enough time for GPU side worst video/audio decode/schedule/render.
This should never be more than 200ms.

This makes playback start and seeks 600ms more responsive which is noticable when seeking through a video.
Revert "[rbp] Always use the aspect ratio from file."

This reverts commit 7f5d3ede514f30552e42464c29227d570b38ed8a.
[rbp/omxplayer] Use aspect ratio from video decoder

We don't get a valid aspect ratio from tvheadend with mpeg2 files, but the decoder does know, so use that