Renderer: Handle resize events on-demand instead of polling

We now differentiate between a resize event and surface change/destroyed
event, reducing the overhead for resizes in the Vulkan backend. It is
also now now safe to change the surface multiple times if the video thread
is lagging behind.

Source/Core/DolphinQt2/RenderWidget.h

@@ -23,7 +23,7 @@ signals:
   void Closed();
   void HandleChanged(void* handle);
   void StateChanged(bool fullscreen);
-  void SizeChanged();
+  void SizeChanged(int new_width, int new_height);
 
 private:
   void HandleCursorTimer();