fix [#35555] Collada: export destroys mesh in some cases

add arguments to calculate normals when converting to bmesh:
  BM_mesh_bm_from_me, DM_to_bmesh

This gives some speedup to undo (which didnt need to re-calculate vertex normals), and array modifier which doesnt need to calculate face normals at all



git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57208 954f8c5b-7b00-dc11-b283-0030488c597c