Additional GridCube support methods and access in SpacePartitioningGrid.
Implemented taxicab distance in Vect3D.
Added thread pool to ThreadManager.
Incremental implementation of new texturing system.
Initial creation of GridCubeProximitySorter.
Implemented multi-threaded visibility updates. Now using reverse
painter's algorithm by sorting GridCubes prior to inserting to rendering
list to improve primary fragment shader performance.