libcore|Matrix: Added Matrix4::frame()
Refactor|libgui|ModelBank: Storing user data in addition to the model
Model Renderer: Model's coordinate system, generic model shader

The model definition may specify a custom coordinate system for the
model, as front/up vectors. By default, a model is automatically
scaled so that its height matches the mobj's height.

"model.skeletal.normal_emission" is (or will be) a generic shader for
per-pixel lit models.