mxfdec: set extradata size

The line setting it was mistakenly removed in
ecf442a58b09bdb1dc1d2c3904b82ac5f79b2878
mpeg4videodec: add a mpeg4-specific private context.
mpeg4videodec: move MpegEncContext.shape to Mpeg4DecContext
mpeg4videodec: move MpegEncContext.time_increment_bits to Mpeg4DecContext

The field remains in MpegEncContext for use in the encoder.
mpeg4videoenc: write 0 instead of vol_sprite_usage

That field is never set for encoding.
mpeg4videodec: move MpegEncContext.vol_sprite_usage to Mpeg4DecContext
mpeg4videodec: remove write-only sprite variables from MpegEncContext
mpeg4videodec: move MpegEncContext.rvlc to Mpeg4DecContext
h263dec: remove commented out cruft
mpeg4videoenc: don't set MpegEncContext.resync_marker

It is only used in one place immediately after it is set.
vc1: move MpegEncContext.resync_marker into VC1Context.

The field still remains in MpegEncContext because it is used by the
mpeg4 decoder.
mpeg4videodec: move MpegEncContext.resync_marker into Mpeg4DecContext.
mpeg4videodec: move mpeg4-specific bug detection from h263 generic code to mpeg4
mpeg4videodec: move mpeg4-specific post-frame-decode code from h264dec to mpeg4videodec
mpeg4videodec: move mpeg4-specific bug workaround variables from MpegEncContext to Mpeg4DecContext
mpeg4videodec: move cplx_estimation_* fields from MpegEncContext to Mpeg4DecContext
mpeg4videodec: move t_frame from MpegEncContext to Mpeg4DecContext
mpeg4videodec: remove a write-only variable from MpegEncContext