Improve dav1d_parse_sequence_header()
This avoids initializing a separate decoder instance, which not only gets rid of a bunch of unnecessary short-lived memory allocations but also makes things three orders of magnitude faster:
2494821 decicycles in dav1d_parse_sequence_header, 62 runs, 2 skips
1551 decicycles in dav1d_parse_sequence_header, 62 runs, 2 skips