AV2 request: stream should ideally start with magic number

[leo-barnes]

During the last SG-image meeting, the fact that an AV1 bitstream is not easily identifiable as such was discussed. Almost all image codecs start with some kind of magic number (JPEG 0xFF 0xD8, PNG .PNG, BMP BM). This makes it easy to embed these codecs inside other container formats without explicitly specifying the codec since a parser can easily identify the codec directly from the first N bytes.

It might be beneficial if AV2 started with some kind of unique magic number. This could be an optional mode that could be used only in specific container formats (or when written without a container) in case we don't want to always have it present.