radiant/image: load fallback images with png loader even if game does not support...

radiant/image: load fallback images with png loader even if game does not support png, fix #141

Current implementation has a minor issue, when "Hide Fallback" option is
disabled, the fallback image does not appear in texture browser before radiant
makes use of it (for example creating a brush before selecting a texture).

Since "Hide Fallback" option is enabled by default and it never makes sense
to apply those fallback intentionally, I don't care for the moment.

I would like to see a better implementation that would work for all
editor images (like the one set with `qer_editorImage` shader keyword).