Fix #7004: Mark linkgraph dirty to be rebuilt on next draw call. (#7265)

Previously the linkgraph was rebuilt before the viewport extents were finalized.

src/main_gui.cpp

@@ -267,7 +267,7 @@ struct MainWindow : Window
 			return;
 		}
 
-		this->viewport->overlay->RebuildCache();
+		this->viewport->overlay->SetDirty();
 		this->GetWidget<NWidgetBase>(WID_M_VIEWPORT)->SetDirty(this);
 	}