movdec: Parse the dvc1 atom

Normally, the actual payload data contains sequence headers, too,
and the parser can extract this and set it as extradata. However,
the data in the dvc1 atom is the "official" extradata for the file.

This is required for proper stream copy of vc1 from ismv to ismv.

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