Blender Git Loki

Add DNA and customdata entries for paint masks.

CD_PAINT_MASK is a layer of per-vertex floats for non-multires
meshes. Multires meshes use CD_GRID_PAINT_MASK, which is a layer of
per-loop GridPaintMask structures. GridPaintMask is similar to MDisp,
but contains an array of scalar floats.

Note: the GridPaintMask could be folded into MDisp, but this way
should be easier to add mask layers in the future (if we do fold
GridPaintMask into MDisp, the mask array should probably be an array
of arrays with a 'totmask' field so that mask layers can be easily
supported.)

Includes blenload read/write support for CD_PAINT_MASK and
CD_GRID_PAINT_MASK.