don't conflate SetImageColorspace and TransformImageColorspace

rmagick is conflating SetImageColorspace and TransformImageColorspace. This is the difference between "convert foo.png -set colorspace rgb ..." and "convert foo.png -colorspace rgb ..."

The former is like a reinterpret-cast in C++: it doesn't change the data, it just says the data is the given colorspace. The latter is a transform that changes the data based on the original image colorspace and the parameter.

I need SetImageColorspace and in the current code, it's not available if TransformImageColorspace is available.

This is a partial patch. I didn't wire in or test image.transform_colorspace(colorspace). If you're interested, I can ...