[Bf-committers] New BMesh API to set normals

[Bastien Montagne]

Hi devs,

Today Howard requested the ability to set custom normals in BMesh (Edit 
mode tools). While we are far from ready to have a real normal editing 
feature/mode/whatever in Edit mode currently, I made a quick port of 
BKE_mesh_evaluate code to set normals, in the new `custom-normals-bmesh` 
branch for now.

As with non-bmesh code, currently you can only set all normals (either 
per-loop or per-vertex) at once, since this is not a cheap operation.

Also added a python bmesh.utils.custom_normals_set() api func, at least 
to allow easy testing of the feature, e.g. with default cube in Edit mode:

 >>> import bmesh
 >>> bm = bmesh.from_edit_mesh(C.edit_object.data)
 >>> nors = sum(zip(((0, 0, 1),) * 4, (None,) * 4), ())  # Produces an 
8-items tuples of Z-up vectors and None's
 >>> # Set those vertex custom normals into bm, 'None' ones keeping 
default auto-computed normals.
 >>> bmesh.utils.custom_normals_set(bm, nors)
((0.0, 0.0, 1.0), (0.5773502588272095, -0.577350378036499, 
-0.5773503184318542), (0.0, 0.0, 1.0), (-0.5773501396179199, 
0.5773504972457886, -0.5773503184318542), (0.0, 0.0, 1.0), 
(0.5773500204086304, -0.5773505568504333, 0.5773503184318542), (0.0, 
0.0, 1.0), (-0.5773501992225647, 0.5773503184318542, 0.5773503184318542))

Testing and feedback is most welcome!

Cheers,
Bastien