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
ee701baff8