Blender Git Loki

Big commit with initial support for sculpting of meshes with the multires modifier.

With this commit, the displacement custom data is being applied to a subdivided version of the base mesh using a modification of CCGSubsurf.

Sculpting on the highest level of the multires mesh now works, with (at least) these caveats:
* Only the plain 'Draw' brush works for now
* Normals aren't being updated during sculpting, so it doesn't actually look right yet
* When displacements are applied or calculated, only interior face points are handled, so original vertices and edge vertices will look wrong after switching levels.