Rework the rendering model so that clients instantiate backing stores.

This makes interactive resizing work a lot better, althought it's still not
perfect. There are still glitches and unpleasant flashes of zeroed memory.

LibGUI/GWindow.h

@@ -68,6 +68,8 @@ public:
     const GWidget* hovered_widget() const { return m_hovered_widget.ptr(); }
     void set_hovered_widget(GWidget*);
 
+    GraphicsBitmap* backing() { return m_backing.ptr(); }
+
 private:
     virtual const char* class_name() const override { return "GWindow"; }