On a place, Ctrl+T, Fkey would create a quad overlapping the 2 Tris.
Now this case is checked for in a general way - if the bounds of the face are already filled in with faces (that _only_ use these edges-verts), then dont create the face.
This is an option for the 'edgenet_fill' operator, since creating the face isnt incorrect, just not-what-you-want mostly.
added functions
* BM_edge_share_vert - returns shared vert between 2 edges.
* BM_face_exists_multi, BM_face_exists_multi_edge - check if existing faces fill the edge bounds.
* also add BM_ELEM_INTERNAL_TAG so low level functions can tag without conflicting with higher level functions that also rely on tagging elements.
* added BM_elem_flag_set, BMO_elem_flag_set. to avoid 'if(...) enable(); else disable();' all over the place.
* added bmesh_operator_api_inline.c, the header file was getting messy.
* BM_vert_in_face now loops over a vert's faces rather then every vert in the face.
* rename bmesh_radial_find_first_facevert --> bmesh_radial_find_first_faceloop
* rename bmesh_radial_find_next_facevert --> bmesh_radial_find_next_faceloop
* rename BM_vert_collapse_edges --> BM_vert_collapse_edge (only collapses one)
