Add displacement handling

[jalberse]

We currently parse displacement maps, but don't do anything with them.

[jalberse]

Note that this refers to the use of displacement maps when defining a ply Shape in a scene description file. That is the directive which will split edges and displace verts for physical displacement. A displacement texture directive in a material deifnition will create a bump/normal map, depending on the number of channels.

[jalberse]

This should be done directly before the PLY file is decomposed into the TriMesh and BilinearPatchMesh