Blender Git Loki

Undo the front-facing only commit for clay strips brushes, it adds an
attenuation that should really be optional. There's also a minor
performance penalty and all this only for one problematic case. In case
the tool flattens two surfaces, users can manually set the front face
only option. A better non-attenuating way to cull such vertices can be
added later.

Also flatten brush should calculate the flatten plane from the original
vertices or the flattening will not converge for planes offsets
different than zero.

Reported by Michalis Zissiou, thanks!