Clear requested_size whenever window_size is set

Since `requested_size` helps implement a sort of double buffering, it
should be cleared whenever we "commit" a new value.

src/server/frontend_wayland/wayland_surface_observer.h

@@ -63,6 +63,7 @@ class WaylandSurfaceObserver : public scene::NullSurfaceObserver
     void latest_client_size(geometry::Size window_size)
     {
         impl->window_size = window_size;
+        impl->requested_size = {};
     }
 
     /// Should only be called from the Wayland thread