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