Blender Git Loki

Brought back hiding parts of the mesh in sculpt mode in proper 2.5-style. (No more ugly reordering of mesh elements leading to mesh corruption!)

UI changes:
* Ctrl+Alt brings up a border select you can hide a rectangular area of the mesh with.
* Ctrl+Shift is the same, but hides the area outside of the rectangle.
* Clicking without dragging while holding either Ctrl+Alt or Ctrl+Shift will re-show hidden areas.
* Added these three operations to the Sculpt menu.

Hiding areas is done by rebuilding the PBVH and excluding primitives based on whether their AABB intersects the user-selected areas. Note that for multires meshes, the primitives are grids, not faces, so if you are bad and use multires on a plain cube, there are only 6*4 grids that can be hidden.

TODO:
* Applying multires temporarily shows hidden areas. Once you start sculpting they hide themselves again. Same for turning off display of multires modifier.
* Going to multires level zero also shows hidden areas improperly.