also changes the name of the callbacks:
- walk_parent_snap_project_cb to cb_walk_parent_snap_project;
- cb_leaf_snap_vert to cb_walk_leaf_snap_vert;
- cb_leaf_snap_edge to cb_walk_leaf_snap_edge;
- test_vert_depth_cb to test_vert_ray_dist_cb;
- test_edge_depth_cb to test_edge_ray_dist_cb;
and cleanup: