Blender Git Loki

Fix T53551: Weight paint crash when subsurf modifier is not first (master not 2.79).

We can only support painting from subsurf DM in a limited subset of
cases, others (like multiple subsurf, or topology-modyfying ones,
break mapping to original geometry).

This is not the most ideal fix (ideally, we should always be able to get
a mapping to original geometry from any point in modifiers stack...).