Just like GdkToplevel::compute-size, the size argument of the signal is
given to the handlers by GDK; it's not an out argument meant to be
allocated by the caller.
Similarly to GdkToplevel, GdkDragSurface's compute-size should be called
by backends to query the current surface size, and should be connected
to by widget implementations (like GtkDragIcon) to report the current
size.
GdkDragSurface-backed widgets are not parented to an existing widget,
unlike popovers, and like toplevels. This means that there's nobody to
actively call gdk_drag_surface_present() to update the size, and
GdkDragSurface should do it on its own, just like GdkToplevel.
