Add the following NAL unit parsing utility functions that will be needed for the MV-HEVC support as proposed in Apple's HEVC stereo video interoperability profile:
- NAL unit header parsing to get the layer information needed for MV-HEVC support.
- VPS parsing, including vps_extension() needed for MV-HEVC support.
- SPS parsing modifications to support MV-HEVC.
PiperOrigin-RevId: 647329211