Redraw rails when zooming

include/app.hpp

@@ -141,6 +141,7 @@ struct RackWidget : OpaqueWidget {
 
 	Widget *onMouseMove(Vec pos, Vec mouseRel) override;
 	void onMouseDownOpaque(int button) override;
+	void onZoom() override;
 };
 
 struct RackRail : TransparentWidget {

src/app/RackWidget.cpp

@@ -524,5 +524,10 @@ void RackWidget::onMouseDownOpaque(int button) {
 	}
 }
 
+void RackWidget::onZoom() {
+	rails->box.size = Vec();
+	Widget::onZoom();
+}
+
 
 } // namespace rack