Using BMesh operators through the edit-mesh API created a full copy
of the mesh so it was possible to restore the mesh in case
one of the operators raised an error.
Remove support for automatic backup/restore from the EDBM_op_* API's
as it adds significant overhead and was rarely used.
Operators that need this can use the BMBackup API to backup & restore
the mesh in case of failure.
Add warning levels to BMO_error_raise so operators can report problems
without it being interpreted as a request to cancel the operation.
For high-poly meshes creating and freeing a full copy is an expensive
operation, removing this gives a speedup of ~1.77x for most operators
except for "connect_verts" / "connect_vert_pair"
which still uses this functionality.
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