Ignore descriptive properties associated after non-descriptive ones

[y-guyon]

Section 6.5.1 of HEIF says:

Readers shall allow and ignore descriptive properties following the first transformative or unrecognized property, whichever is earlier, in the sequence associating properties with an item.

• I do not think libavif respects that constraint.
• I am not sure what is the definition of an "unrecognized property".
• I guess "ignoring a descriptive property" means treating it like it was not there.
  Because descriptive properties should not impact the rendering of the decoded image, the effect of ignoring one can only be:
  • treating the file as invalid from a missing mandatory descriptive property (such as ispe),
  • not exposing the same image metadata from the libavif API.

[bradh]

I think there could be a distinction between ignoring a property at the application level and ignoring it at the library level. Or maybe this needs to go to MPEG?