Add tangent space mode for vertex and weight paint brush

Painting in Blender is really nice but I miss a feature that was available in Softimage and that made the experience even better.

When painting a mesh in Softimage, the "paint" would propagate in tangeant space in such way that only connected vertex would be affected by the radius. This behavior was extremely pleasant to work with and avoided any sort of undesired bleeding that can happen from a brush operating in screen space.

You can sort of see what I mean on this video even though the demonstrator actually does not take advantage of that feature and repositions the character. :D