Blender Git Loki

Add mask-drawing support to GPU_Buffers.

This is the last commit of the sculpt masking merge. Documentation:
http://wiki.blender.org/index.php/User:Nicholasbishop/PaintMasks

Thanks to Brecht for reviewing!

* For VBO, add color to the VertexBufferFormat structure as three
unsigned bytes. Since mask elements are scalar the three color
components are identical to eachother, but the fixed-function OpenGL
pipeline requires colors to be either three or four components.

* For the same reason, multires VBO drawing now copies into the
VertexBufferFormat format as well.

* Regression: material colors will not show up correctly now, masks
colors are overriding. Not sure how to fix this nicely (would be
much easier to fix if drawing with vertex shaders.)

* Also, masks will only draw PBVH drawing, so only 'solid' drawing
will work correctly with masks.