Blender Git Loki

Workbench: Force Vertex Colors in Paint Mode

Vertex colors behaved differently as the paint overlay mixed the colors
in display mode and the results was multiplied on top of the original
shading.

This patch will align the implementation to texture painting where the
colors are drawn by the workbench engine so the correct shading is
applied.

This also means that we don't show the vertex colors overlay when not
in solid mode.

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D6436