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/Host.h

@@ -26,7 +26,7 @@ public:
   void SetRenderHandle(void* handle);
   void SetRenderFocus(bool focus);
   void SetRenderFullscreen(bool fullscreen);
-  void UpdateSurface();
+  void ResizeSurface(int new_width, int new_height);
 
 signals:
   void RequestTitle(const QString& title);