xcf: implement support for v12 (various bit depths and floating point), improve colorspace handling and more blending

With this there's no .xcf files left without thumbnails here at least. Also attempts to use QPainter's composition modes in more cases (it's faster and the results are closer to gimp's according to diffimg).

For floating point it uses the new native format in qt 6.2 if available, or crushes it down to 16bit ints.