Blender Git Loki

Sculpt: Use uchar to store the sculpt mask in the GPU

Using a float to store and render the mask seems like a waste of memory
without any noticeable difference in the viewport for its use case.
After this commit, the mask and the face sets combined should take the
same amount of GPU memory than only the mask in previous versions.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D7148