Add third pass for terrain paint layer nodes

[cmbasnett]

We currently have two "passes" for terrain painting. First, the layer's paint layer nodes are evaluated, then the doodad paint layers are evaluated. However, there is no way for the user to add any node evaluation after this.

If we had a third pass, the user would be able to globally modify the results from the doodad evaluation.

I might need some more convincing that this is worth doing, since it would require a pretty large amount of work and maintenance cost.