Sculpt mode drawing fails after deleting a subsurf modifier
in sculpt mode and undoing.
This was quite difficult to spot. Main cause was that mesh data was not
synchronized properly between undo and sculpt code because we generated
a pbvh on derivedmesh invalidation without really refreshing the rest of
the data. This could result in undo and drawing operating on different
data.
To solve this and avoid bad level calls I had to move quite some code
around. Crazyspace is now moved to blenkernel, as did some sculpt calls
that make sure sculpt data are properly refreshed.
