Add overlay support to the model

[Gudahtt]

Each chunk can now define an overlay. An overlay is drawn over the base model, and can have its own color, gradient, and mask properties. An overlay can be used to apply a pattern over the entire model at once.

Using an overlay does have a performance impact because each polygon in a chunk with an overlay gets duplicated. But it makes it dramatically easier to apply a pattern over the entire model at once.

A demo of this functionality has been added, called "overlay". It is not the nicest looking example, but it does demonstrate the functionality well.

[Gudahtt]

~This depends upon #76~