gdk/toplevel: Make gdk_toplevel_present() async

gdk/toplevel: Make gdk_toplevel_present() async

The plan is to concencrate size computations as part of the frame clock
dispatch, meaning we shouldn't do it synchronously in the present()
function.

Still, in Wayland, and maybe elsewhere, it is done in the present()
function, e.g. when no state change was made, but this will eventually
be changed.