Blender Git Loki

Add first version of pressure influence for mask textures based on
feedback from David Revoy:

http://www.davidrevoy.com/article107/textured-brush-in-floss-digital-

There are two methods, cutoff and ramp.

Cutoff simply selects between 0 and 1 based on stylus pressure while
ramp distributes the mask effect above the pressure value.

Too many words, here is a screenshot with the effect:

http://www.pasteall.org/pic/show.php?id=53865

From top to bottom:
* Strength pressure influence ON:
- No mask influence
- Cutoff mask influence
- Ramp mask influence
* Strength pressure influence OFF:
- No mask influence
- Cutoff mask influence
- Ramp mask influence

The brush masking is still in need of attention because it behaves
erratically, still it's nice for an zero.alpha commit ;)