vc1parse: fix malformed sequence layer header and STRUCT_C

vc1parse: fix malformed sequence layer header and STRUCT_C

This commit fix several issues with sequence layer header forging on
update_caps():
- 0x00000004 unsigned integer is before STRUCT_C.

- Set reserved bits of STRUCT_C to their values for simple/main
  profiles in sequence layer header format and ASF header format.

- Sequence layer shall be represented as a sequence of 32 bits unsigned
  integers and shall be serialized in little-endian byte order except
  for STRUCT_C which shall be serialized in big-endian byte-order.

See SMPTE 421M Annex L for more details about sequence layer format.

https://bugzilla.gnome.org/show_bug.cgi?id=736474