Essential item properties should be checked

[bradh]

There is a flag on item property references if it is "essential" that it be understood. Right now, it looks like we're ignoring that, and potentially trying to handle images that require functionality we don't provide.

I hit this experimenting with the ISO 23001-17 Amd 2 CDAM generically compressed image support, where there are some extra properties to say "zlib inflate the data before decoding".