Change #10255: Reduce basic thickness of linkgraph GUI lines (#10410)

From 3px to 2px (multiplied by UI scale).

src/main_gui.cpp

@@ -224,7 +224,7 @@ struct MainWindow : Window
 		NWidgetViewport *nvp = this->GetWidget<NWidgetViewport>(WID_M_VIEWPORT);
 		nvp->InitializeViewport(this, TileXY(32, 32), ScaleZoomGUI(ZOOM_LVL_VIEWPORT));
 
-		this->viewport->overlay = new LinkGraphOverlay(this, WID_M_VIEWPORT, 0, 0, 3);
+		this->viewport->overlay = new LinkGraphOverlay(this, WID_M_VIEWPORT, 0, 0, 2);
 		this->refresh.SetInterval(LINKGRAPH_DELAY);
 	}