Mesh: Optimize vertex to edge attribute domain interpolation

Instead of using `DefaultMixer`, use the simpler `mix2` function.
This just decreases the overhead of computing each value.
In a simple test storing an attribute with the position of each
edge, I observed a 1.7x performance improvement: a change from
16 ms to ~9ms for an 8 million edge mesh.

Resolves #133196.

source/blender/blenkernel/intern/mesh_attributes.cc

@@ -414,13 +414,8 @@ static GVArray adapt_mesh_domain_point_to_edge(const Mesh &mesh, const GVArray &
       else {
         new_varray = VArray<T>::ForFunc(
             edges.size(), [edges, varray = varray.typed<T>()](const int edge_index) {
-              T return_value;
-              attribute_math::DefaultMixer<T> mixer({&return_value, 1});
               const int2 &edge = edges[edge_index];
-              mixer.mix_in(0, varray[edge[0]]);
-              mixer.mix_in(0, varray[edge[1]]);
-              mixer.finalize();
-              return return_value;
+              return attribute_math::mix2(0.5f, varray[edge[0]], varray[edge[1]]);
             });
       }
     }