HeightMap 2

+ Fixed the tick-squash related discrepancies between Histogram and
HeightMap.
+ Heightmap heights now sum to the size of the cube, instead of scaling
to the top. The shader coloring scales with the new max height.
HeightMap 3

- Removed commented out code
+ Fixed issue where selecting a single attribute for the Z axis would
result in the heightmap becoming a cube.
+ Heightmap is now correctly disabled when not being used.