comparing bmesh to trunk, mesh_calc_normals() in bmesh is a much more comprehensive function, calculating mpoly,mface normals, where trunk only calculated vertex normals.
renamed:
* mesh_calc_normals() --> mesh_calc_normals_mapping_ex
* mesh_calc_tessface_normals --> mesh_calc_normals_tessface() - only calculates normals from tessface
* added mesh_calc_normals() - only calculates normals from poltys
this way we can have mesh_calc_normals() remain fast for parts of the code which only need vertex normals to be updated.
only refactor, no func changes- didnt replace mesh_calc_normals_mapping_ex() with mesh_calc_normals() anywhere yet.