Blender Git Loki

More tweaks related to bump mapping quality;

While going over the code, I found out the "nabla", the size of offset
vectors for calculating derivatives of a texture, is a built in constant.
Even worse, the value was different for new noise types (musgrave etc).
So I've added a new slider for it in the procedural texture panels, which
by default is set to 0.025, the value of the old constant. Also made sure
it works with equal effect in all procedurals.

NOTE: a small Nabla will give sharper, detailed bump, but the effect also
becomes smaller, correct that in the Mapping Panel of materials.

For better & compliant control over the bumpmapping, I've also included
the Colorband output in derivatives calculus, so the bump output then
matches the color created. It's also a nice tool to finetune output of
textures for bumpmapping in general.

Bug fix; clicking on the rightmose 'item' in ColorBand didn't activate it.
Found out the ColorBand was slightly drawn off (2 pixels).