uncompressed: support version 1 boxes

[bradh]

This adds support for parsing uncompressed heif files using version 1 of the uncC box, which is a shortcut for simple RGB, RGBA and ABGR files at 8bpp.

I looked at trying to synthesize the extra box (cmpd) at parse time. That didn't work out, because the item property association then didn't line up. Instead, the extra boxes are produced when the item properties are requested.