Model importer and processor improvements

I've addressed some bugs with model importing and processing, now the output looks much closer to XNA. It can properly build the dude and ship assets from XNA samples, so long as the source files are updated to the newer FBX versions first. This should also resolve #2182, which had a missing triangles problem.

I also filled in some of the gaps for material processing necessary to get things building and models written out, so the ModelProcessor properly chains to build materials and textures.