Investigate generating materials at editor time, rather than using an uber-shader

[kring]

Our default material for rendering tilesets does a lot of stuff. It has nodes that support:

1. every glTF material feature
2. three raster overlays
3. a clipping layer

And there's a separate material that adds:

4. water

This all makes it quite a heavy material, even when the tileset we're rendering doesn't use most of these features.

Creating materials at runtime is probably hard/impossible, but we still might see significant advantages from generating a material for a tileset at editor time. Much like Nithin did for metadata styling in #698.

Related to #287.