Now create the rotate edge in advance and copy its customdata (before joining the faces).
This commit also fixes an annoyance where tryangulating faces could create duplicate edges.
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
* improve check to see if edge rotate can be done,
was checking if both edges verts have an edge count of 2, which is really a meaningless test since the verts can have stray edges connected and the result wont work right.
instead check if the next verts in both faces share a vertex.
* add utility function BM_face_other_vert_loop() which gets the next loop in a face.
* add convenience function BM_edge_face_pair() which returns 2 faces for edges that have exactly 2 face users. (saves ugly e->l->radial_next ... in code) and is more readable.
