Multiple materials per shape.

[xelatihy]

Shapes can only contain one material now. This is limiting since we need to split shapes on IO.
The simplest fix is that allow multiple materials in shapes and subdivs.

[xelatihy]

After some implementation trial, this turned out to be very hard to do with idiosyncrasies in the renderer. What seems better is to split the concept of instance with single shape and single material and surface with multiple materials. The real question is where to put these. One option is to put these inside nodes or inside a new instance type. But this needs to be worked on more.