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.

QRgb is a typedef of **unsigned int**.  The red, green, and blue parts are the second, third, and fourth most significant bytes.  So this hash algorithm generates a collision whenever blue and green are both equal to 31.  I think you should be using two distinct prime numbers, both of which are greater than 256 (i.e. 257 and 263).