Topology Genus Change.

This feature allows carving holes while sculting in dyntopo.

Part 1 of the feature is implemented here, it just deletes the vertices
when they are close together. The union part is coming soon.
WIP commit
Yet more WIP. Now also delete unused vertices.
More WIP, move some code around
More WIP.
WIP: refactor, use lists to sort through potential merged vertices, also
factor out unneeded function (logic was needed in only one place after
all)
Even more WIP code. Many crashes here, I will need to take care of stray
geometry.
Yet more WIP
At last much more stable code
Yet more fixes
Yet another stable snapshot.
WIP, debug stuff
Add asserts, do fill 'correctly' (still fails spectacularly though)