Big metavoxel update: removed unused pieces, converted heightfields to "spanners" in order to allow arbitrary transforms, switched to 16 bit heightfields with lossless compression, some rendering improvements.

Sorry.  What I should have said: "There is a known collision between (r,g,b) = (r,31,1) and (r,0,32)."  Using two distinct primes larger than 256 guarantees that there are no collisions, and we also know that the hash will never exceed the max value of "unsigned int", so no truncation.