[rbp] Fix for stuttery video when seeking before zero

There are a few issues with seeking I found.
We weren't correctly setting OMX_BUFFERFLAG_TIME_UNKNOWN on the first frame after a seek which could make the GPU think video was at 0 and audio at a much larger offset.
A full video fifo (to GPU) stops any higher priority messages from being received which can stall a seek and the flush message doesn't get through. Use m_flush to discard the video packet that doesn't fit.
We get an audio frame through with unknown pts/dts after the flush, but before the GENERAL_RESYNC when seeking. This was given to GPU and was perhaps 30 seconds out from the following packets and that throws off the timing between audio and video streams. Keeping m_flush true until the GENERAL_RESYNC discards this frame. Hopefully that is safe.
Merge pull request #2368 from popcornmix/seek_before_zero

[rbp] Fix for stuttery video when seeking before zero