Add some more properties to the substrate

We now store the paint-color in a cell separately to the intrinsic color
of a cell. The renderer will prefer the paint color if it is set on a
cell.
Add paint aging function
Add rgb <-> hsl conversion functions
Create mixing function and support
Switch brushes to mix with substrate
Add multiple brushes and switching with num keys
Make extract and patch work on variable sizes
Make the brushes bigger