This byte represents 4 reserved bits followed by 4 alpha_channel_type bits.
alpha_channel_type currently has 3 differents defined values: 0 (no
alpha), 1 (8b alpha), and 2 (16b alpha), all the other values are
reserved. The 4 initial reserved bits are expected to be 0.
Quoting SMPTE RDD 36:2015:
A decoder shall abort if it encounters a bitstream with an unsupported
bitstream_version value. If 0, the value of the chroma_format syntax
element shall be 2 (4:2:2 sampling) and the value of the
alpha_channel_type element shall be 0 (no encoded alpha); if 1, any
permissible value may be used for those syntax elements.
So if we're not in 4:2:2 or if there is alpha, we are not allowed to use
version 0.
