movdec: Restart parsing root-level atoms at the right spot

movdec: Restart parsing root-level atoms at the right spot

If parsing moov+mdat in a non-seekable file, we currently
abort parsing directly after parsing the header of the mdat
atom. If we want to continue parsing later (if looking to
parse later fragments), we need to skip past the content of the
mdat atom, otherwise we end up parsing the content of the mdat
atom as root level atoms.

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