Refactor CDData masks, to have one mask per mesh elem type.

We already have different storages for cddata of verts, edges etc.,
'simply' do the same for the mask flags we use all around Blender code
to request some data, or limit some operation to some layers, etc.

Reason we need this is that some cddata types (like Normals) are
actually shared between verts/polys/loops, and we don’t want to generate
clnors everytime we request vnors!

As a side note, this also does final fix to T59338, which was the
trigger for this patch (need to request computed loop normals for
another mesh than evaluated one).

Reviewers: brecht, campbellbarton, sergey

Differential Revision: https://developer.blender.org/D4407

source/blender/modifiers/intern/MOD_mirror.c

@@ -95,7 +95,7 @@ static Mesh *doBiscetOnMirrorPlane(
 		&(struct BMeshCreateParams){0},
 		&(struct BMeshFromMeshParams){
 			.calc_face_normal = true,
-			.cd_mask_extra = CD_MASK_ORIGINDEX,
+			.cd_mask_extra = {.vmask=CD_MASK_ORIGINDEX, .emask=CD_MASK_ORIGINDEX, .pmask=CD_MASK_ORIGINDEX},
 		});
 
 	/* Define bisecting plane (aka mirror plane). */
@@ -122,7 +122,7 @@ static Mesh *doBiscetOnMirrorPlane(
 		}
 	}
 
-	result = BKE_mesh_from_bmesh_for_eval_nomain(bm, 0);
+	result = BKE_mesh_from_bmesh_for_eval_nomain(bm, NULL);
 	BM_mesh_free(bm);
 
 	return result;