Modifier stack: Fix broken 'dirty normals' flag for new mesh-aware modifiers.

Since modifier stack is still primarily based on DM in blender2.8, we
need to copy over 'dirty normals' flag from temp Mesh to DM after
modifier has been evaluated.

source/blender/blenkernel/intern/cdderivedmesh.c

@@ -611,6 +611,11 @@ DerivedMesh *CDDM_from_mesh_ex(Mesh *mesh, int alloctype)
 	dm->deformedOnly = 1;
 	dm->cd_flag = mesh->cd_flag;
 
+	if (mesh->runtime.cd_dirty_vert & CD_MASK_NORMAL) {
+		dm->dirty |= DM_DIRTY_NORMALS;
+	}
+	/* TODO DM_DIRTY_TESS_CDLAYERS ? Maybe not though, since we probably want to switch to looptris ? */
+
 	CustomData_merge(&mesh->vdata, &dm->vertData, mask, alloctype,
 	                 mesh->totvert);
 	CustomData_merge(&mesh->edata, &dm->edgeData, mask, alloctype,