movenc: Use the actual converted sample for RTP hinting

If an annex b bitstream is muxed into mov, the actual written
sample is reformatted to mp4 syntax before writing.

Currently, the RTP hints that copy data from the normal video
track, where the payload data might be offset compared to the
original sample that the RTP hinting used (when 3 byte
annex b startcodes have been converted into 4 byte mp4 format
startcodes).

Signed-off-by: Martin Storsjö <martin@martin.st>