Clarify PNG gamma correction

[curiousdannii]

I think some clarifying statements on PNG gamma correction would help. I don't think they should necessarily be hard rules; there may be some platform or programming language where removing gamma correction might not be supported out of the box or is just flat out impossible. But they should be strong recommendations.

They should cover both sides: interpreter authors are strongly recommended to ignore the gAMA chunks of PNGs and output all images as well as primitive rectangles with a uniform gamma. Story authors are encouraged to remove gAMA chunks. If there is a cross-platform command/tool (pngout? imagemagick?) which can remove the chunk then very brief instructions could be helpful.

(I wonder if it would be feasible for Inform 7 to put all images through a compressor and cleaner before building the blorb? That's something for the I7 suggestion list though.)