AV2 request: metadata section to store a string for encoding information

[1480c1]

Currently, both x264 and x265 support writing a string into their SEI headers that contain a simple key=value space separated string that contains information related to encoder specific settings (see x264_param2string() for what x264 does for generating the string, and x264_sei_version_write() for the full string it writes, similar thing with x265_param2string()) and this can really help when trying to determine what might be the difference between two nearly identical bitstreams.

This format is obviously not necessarily standardized, as not all h264 and h265 encoders will put the same information, but some common open source ones do (for example kvazzar does so in encoder_state_write_bitstream_prefix_sei_version())

A cursory glance at the spec does not seem to hint at any place where it might be possible to insert such a string into the av1 bitstream, whether that would be in the obu metadata header or somewhere else for encoder specific settings, however, someone did mention that perhaps it may be possible to insert such information in a padding OBU https://gitlab.com/AOMediaCodec/SVT-AV1/-/issues/1912

[BlueSwordM]

In my opinion, you'd have 4 states for how detailed the metadata could:

0. Disabled.
1. Command line encoder settings(default).
2. Command line encoder settings + enhanced all preset speedfeature settings (enhanced).
3. Command line encoder settings + all preset speedfeature settings + quantizer/frame type for each frame(development grade).

That way, it could be very easy to identify what coding features/settings/frame information just by looking at somewhat human readable metadata.