Blender Git Loki

Fix weight paint Blur, Smear and Average breakage in multi-paint mode.

This is most pronounced in Auto-Normalize + Multi-Paint. Unlike
vertex paint, the weights being painted on in weight paint mode
don't necessarily correspond to the weight actually stored in
any one vertex group, and may instead be a computed aggregate.

This restores original code behavior lost in rB4f616c93f7cb.