New

Kept plugging away and added options for new images and new layers at last.

Not all of the layer types work 100% yet, but they work for normal drawing and layer operations and PNG at least saves mono images as mono. I probably don't need them all there - I just did it because I could.

All the calculations and blending is just done in float RGBA so none of the code needed changing, although it does mean it's doing a lot of redundant work if the result is only monochrome.