Updates which decouple the physics from the rendering and unlock the
frame rate. I'm using the "Fix Your Timestep" model, but I haven't
yet figured out how to do the interpolation for the final form of
the timestep fixes on that page because my state is a radically
different representation than what that page assumes.  I still need
to wander through the source and ensure that various temporal values
are represented properly in usec and such.