Blender Git Loki

Sculpt: add buttons in sculpt header to toggle
vertex color drawing.

* Added two buttons to toggle between vertex
and material color modes in workbench drawing
mode.
* They are labeled "Colors" and "Normal" to
avoid confusion; "Vertex" is ambiguous, while
"Material" might be taken to impley "EEVEE."
* For now they only show up if a sculpt paint
brush is active.
* Updated startup.blend so the sculpt layout
has vertex drawing mode on by default.