Sticking a full frame in the extradata works, as the code for writing
the avcC/hvcC extracts the relevant parameter set NAL units - provided
that they actually exist in the frame.

Some encoders don't provide split out extradata directly on init (or
at all). In particular, the MediaFoundation encoder wrapper doesn't
always (depending on the actual encoder device) - this is the case for
Qualcomm's HEVC encoder on SD835, and also on some QSV H264 encoders).

This only works for cases where the moov hasn't already been written
(e.g. when not writing fragmented mp4 with empty_moov, unless using
the delay_moov option).
Signed-off-by: Martin Storsjö <martin@martin.st>