Blender Git Loki

fix for a bmesh glitch when making a face (Fkey).

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.