various: update Qt code deprecated in 5.15

- Replaces QLinkedList with std::list as recommended by Qt docs.
- Replaces QWheelEvent::delta() with QWheelEvent::angleDelta(). This
  should also benefit trackpad behavior.

app/render/backend/renderbackend.h

@@ -102,7 +102,7 @@ private:
 
   QThreadPool pool_;
 
-  QLinkedList<RenderTicketPtr> render_queue_;
+  std::list<RenderTicketPtr> render_queue_;
 
   struct WorkerData {
     RenderWorker* worker;