Align definition of config record with AV1 MPEG2-TS

[cconcolato]

The current draft of the MPEG-2 TS binding spec for AV1 reuses the config record, but uses 2 of the reserved bits for HDR signaling. The ISOBMFF spec should be updated to make sure these 2 reserved bits will not be reused, and maybe to indicate that they are ignored in ISOBMFF context.

[cconcolato]

We should make sure that the behavior of encoder/decoder is clear with respect to the reserved=0 part.

• Should an AV1-ISOBMFF encoder reusing data from an MPEG-2 TS stream reset the bits to 0? or can it leave them as is?
• Should an AV1-ISOBMFF decoder ignore the bits, and not complain if they are not 0? Should we remove = 0?

[cconcolato]

Given that the bits in MPEG-2 are signaling HDR which can be signaled elsewhere in ISOBMFF, we probably should force the bits to be 0.

[cconcolato]

Maybe the value 0 in MPEG-2 TS should mean no indication is made, so that after editing in the MP4 space, it remains valid to put it back into TS.