Improvements to bmesh edge rotate

On a user level, edge rotate now works better with multiple edges selected, it wont make zero area faces or rotate edges into existing ones.

With a single edge selected - rotate is less strict and will allow ugly resulting faces but still checks on duplicate edges.

API:
* BM_edge_rotate now takes a flag, to optionally...
** check for existing edge
** splice edge (rotate and merge)
** check for degenerate resulting faces (overlapping geometry, zero area)
** beauty - only rotate to a better fit.
... this allows it to still be used as a low level API function since all checks can be skipped.

* BM_edge_rotate() now works a bit different, it find the new edge rotation before joining the faces - exposed by BM_edge_rotate_calc().

* Added api call bmesh_radial_faceloop_find_vert() - Radial Find a Vertex Loop in Face

source/blender/python/bmesh/bmesh_py_utils.c

@@ -291,7 +291,7 @@ static PyObject *bpy_bm_utils_edge_rotate(PyObject *UNUSED(self), PyObject *args
 
 	bm = py_edge->bm;
 
-	e_new = BM_edge_rotate(bm, py_edge->e, do_ccw);
+	e_new = BM_edge_rotate(bm, py_edge->e, do_ccw, 0); /* BMESH_TODO - expose to API */
 
 	if (e_new) {
 		return BPy_BMEdge_CreatePyObject(bm, e_new);