Notes:
- only cuts through when the occlude geometry button is unpushed,
else the old behavior
- doesn't add points in middle of occluded faces yet, so will not
make partial cuts or hole cuts in occluded faces; for best
results, start and finish cuts completely outside the mesh
- also made slight improvement to behavior when starting cut
off the model
- a small refactor: added knife_add_to_vert_edges for code
used a number of times
7ee3da151d