Images are construction by default

With this change and the previous commit images will not be included in
3d groups by default.

src/mouse.cpp

@@ -1138,6 +1138,7 @@ void GraphicsWindow::MouseLeftDown(double mx, double my, bool shiftDown, bool ct
                     AddToPending(hr);
                     Request *r = SK.GetRequest(hr);
                     r->file = pending.filename;
+                    r->construction = true;
 
                     for(int i = 1; i <= 4; i++) {
                         SK.GetEntity(hr.entity(i))->PointForceTo(v);