Blender Git Loki

Face Sets: Use white color for a default Face Set to enable the overlay

This introduces a variable to store a face set ID which is going to be
rendered white. When initializing a mesh or randomizing the colors, this
variable gets updated to always render a white face set. This way the
face set overlay can be enabled without adding colors to the mesh if
face sets are not in use. After creating the first face set, new colors
are generated randomly like usual.

The face set stored as default does not have any special meaning for
tools or brushes, it just affects the rendering color.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D7035