Optimize BKE_mesh_calc_normals(), gives approx 25% speedup.
- no need to allocate polygon normal array.
- no need to use BLI_array_ functions (realloc's).
- reduce some of the looping.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56649 954f8c5b-7b00-dc11-b283-0030488c597c