Palette swatches gain a new property: Pattern.
Patterns are grayscale textures that the swatch color will sample
against when drawing pixels to the level, by taking the world coordinate
modulo a value inside the texture.
A few algorithms were tried (Screen, Overlay), this branch lands on one
that tries to cast the color from grayscale which comes out rather dark;
to get a patterned color to look black while still seeing the pattern,
the color needs to be as bright as #777 to get the effect.
