This is the beginning of supporting 2d rendering in 3d space. The idea
is that a canvas can be in 2d orthographic space (not attached to any
entity with a 3d transform), or in 3d perspective space (attached to an
entity with a 3d transform, either as a child of the camera, or of some
object in 3d space).
It will replace the current HUD code when it's working.
