Projection layer type

[stephenwf]

So far the layers on the canvas have been relative to the canvas itself, but we may need layers that are relative to the viewer or viewport when it makes sense (such as box selector layer for example). In this case we want a projection painted onto the canvas where the real element is, and a way to relay back the position of the canvas to the this layer.

Top Layer - viewport
Cube - projection layer, moves with viewport
Bottom layer - canvas, relays projected co-ordinates to projection