Added point model visualization to the Viewer program

[Morgan McGuire]

It supports .xyz, .vox, and .points files and is optimized for fast rendering and cached loading of giant point sets. You can fly through the model with the usual G3D first person or game controller controls. The default settings autodetect the file format options of the variations of xyz files (I patched PointModel to implement this, and upgraded PointModel to the latest deferred shading API). 

Models are auto-centered on load but not auto-scaled currently. I'm doing a little more debugging and may implement auto-scaling as well.

-m