Voronwe/gradient brush

The changes here add support for using a brushtip as a Gradient map instead of an Image, Alpha mask, or Lightness map. This will allow for many more options with brush stamps, and some cool effects with regular brushes. To make this more useful, I also added the ability to use Foreground or Background colors in gradients.

This is built on top of my texturingfeatures branch, so it includes all of the texture gradient options I added there.

Everything builds and runs as expected. Existing brushes, textures, and gradients all appear to work as before. This just adds new very useful features for brushes, textures, and gradients.

One note on performance: gradient brushtips are slow unless precision is turned down. I don't see any negative effects with precision:1, and performance is great there, so I recommend that for any brushes using gradient mode. Stamp brushes with high spacing might not need this, but painting brushes can be really slow at precision:5.