A 3D transform associated with the surface.
This transform defines a space which serves as a consistent frame of reference for operations involving the surface, which is particular important for surfaces which can move. For example, consider the screen of a virtual tablet which can be held and manipulated, but also touched and poked. Even as the tablet moves through space, poke interactions on its screen should be calculated from the screen's perspective, not from the world's perspective: if a finger is held still and the tablet moved forward to touch it, from the tablet's perspective the finger moved to the screen, and thus the interaction is a poke. This transform provides the frame of reference that facilitates these sorts of calculations.
