Blender Git Loki

Sculpt: Allow non-manifold collapse

* This was always buggy, which is why
I have code that detects and snips off
non-manifold fins.
* Turns out it is more robust to simply allow
non-manifold collapses and clean up geometry
afterwards. IIRC this is how DynTopo (and the
paper it's based off of) originall worked.