Enable depth testing
Add a frustum projection
Give Primitive a move constructor

The move constructor only works before it has been constructed for real
(i.e. before it has been loaded) I'm not sure if I want to change that,
and allow GLRefs to be move-constructed also.
Add model loading

Models load in multiple primitives from lists of vertices, colors,
texture coordinates, and indices.
Make abxy work better with external projects
Make the ortho2d projection more reliable

Don't have it do any scaling, just have it make sure that the y/x ratio
is the same as the screen.
Add the model exporter blender script

Modify the names a little bit for consistency