restore multithreading on iOS and use DisplayLink to decouple rendering from the main loop

src/node_stroke_preview.cpp

@@ -492,6 +492,7 @@ void NodeStrokePreview::draw_stroke()
                     gl.save();
                     node->draw_stroke_immediate();
                     gl.restore();
+                    node->app_redraw();
                     node->async_end();
                     std::this_thread::sleep_for(std::chrono::milliseconds(30));
                 }