Physics and Drawing Improvements, demos improved

Drawing first: Chunks now have the capacity for multiple textures at a
time, with the same efficiencies as before.  This leads effectively into
having multiple types of tiles (which was already possible, but not
visually easy to see).

Physics: Entities can now interact with each other safely, without fear
of horrible teleportation.

Demo1: Now has ice tiles, to show off the visual distinctions as well as
the classic use of nonstandard friction / traction.