Add a diff tool

[mitchcurtis]

This would be useful for e.g. comparing animation frames. For example, you have two frames that look really similar but you can't be sure, and it's slow to compare everything by e.g. making selections and comparing their sizes. The tool work similarly to crop in how you start using it: first you need to make a selection. Then, click on the diff button to start seeing the difference between the contents of the selection (only applies to image contents in the current layer) and whatever is underneath it. So you'd move the mouse around to find the similar frame and it would show the difference between the two by e.g. partially transparent red pixels. You can cancel it by clicking or pressing escape.

There could also be different diff options:

• Show only different pixels (in red)
• Show different pixels (in red) and identical pixels (in green)
• Show only identical pixels (in green)

The red and green colours might not work for images that are red or green, so we could e.g. darken the canvas once the diff button is clicked to make it stand out more.

[mitchcurtis]

I guess onion skinning support for animations could also achieve this, though a diff tool would be more broadly useful.