Fix sixtyfps::quit_event_loop() not exiting the event loop with Qt 6 on macOS

As per commit 0c02f133f3daee146b805149e69bba8cee6727b2 in qtbase (qt6),
quit() on QCoreApplication on macOS calls [NSApp terminate], which will
not return to main. The latter however is documented behavior, and
sixtyfps-viewer for example relies on it.

sixtyfps_runtime/rendering_backends/qt/lib.rs

@@ -150,7 +150,11 @@ impl sixtyfps_corelib::backend::Backend for Backend {
         {
             use cpp::cpp;
             cpp! {unsafe [] {
-                qApp->quit();
+                // Use a quit event to avoid qApp->quit() calling
+                // [NSApp terminate:nil] and us never returning from the
+                // event loop - sixtyfps-viewer relies on the ability to
+                // return from run().
+                QCoreApplication::postEvent(qApp, new QEvent(QEvent::Quit));
             } }
         };
     }