A problem with the current flag seting in Mesh is that Mesh needs to know of all possible flags or setting a flag can raise an error from the faces own unrecognezed flag.
also stopped the active face flag raising an error so pythoners can do face1.flag |= face2.flag without checking for active face flags. if the flag is a part of the arg its removed quietly.
Checked Mesh flags, face modes and edge flags, should all be ok now.
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