Blender Git Loki

BMesh: remove doubles fix/optimization

Changes to remove doubles face creation,
Recent change to remove doubles broke when the new faces already existed (rare occurrence),
however theres no point to return an existing double face.

Now check if the face exists before creating it.

Other changes:

- avoid 2x hash lookups on all mapped verts.
- fill in the vert array instead of calculating from edges.
- remove inefficient search of entire edge-array before adding to it.
(flag verts to ensure they're not used multiple times).
- move logic for transfusing edge-flags to edge creation.