Workbench: Support Active Vertex Color

Currently it is not possible to view the vertex colors of an object. To
optimize the workflow, workbench will need to support Vertex Colors.

The Vertex Colors is a new option in `shading->color_type`. When objects
do not have vertex color, the objects will be rendered with the
`V3D_SHADING_OBJECT_COLOR`.

In order to support vertex colors in workbench the current texture/solid
shading structure is migrated to a primary shaders and fallback shaders.

Fix: T57000

Reviewers: brecht, fclem

Differential Revision: https://developer.blender.org/D4694

source/blender/draw/modes/sculpt_mode.c

@@ -161,6 +161,7 @@ static void sculpt_draw_mask_cb(DRWShadingGroup *shgroup,
                      false,
                      false,
                      true,
+                     false,
                      (void (*)(void *, struct GPUBatch *))draw_fn,
                      shgroup);
   }