avoid temporary copy

Change-Id: Ica949141c58d7ebb85f246dee4109c39ee8f889e

chart2/source/view/inc/GL3DRenderer.hxx

@@ -141,7 +141,7 @@ struct RoundBarMesh
 struct PackedVertex{
     glm::vec3 position;
     glm::vec3 normal;
-    bool operator<(const PackedVertex that) const{
+    bool operator<(const PackedVertex& that) const{
         return memcmp((void*)this, (void*)&that, sizeof(PackedVertex))>0;
     };
 };