BMesh: optimize edge & face group calculation

This changes the search for unprocessed faces to only search
from the previously found face. Local testing on 1.5 million
triangle meshes gives a 75x speedup
(of the code affected, which is the first half of the work).

The former code would traverse all faces of a mesh until a face was
found that had not been processed. This ended up being slow mainly
because it had to load face-data to determine the state of the flag.
Secondarily, the way it iterated and marked the mesh, it would end up
traversing all previously processed faces to find and unprocessed one.

The same optimization has been made for edge-group calculation.

Reviewed By: campbellbarton

Ref D12379