This is necessary to avoid a deadlock with the compositor. When setting
a stage size before the stage was shown this would trigger a redraw
inside clutter_stage_wayland_resize. This redraw would result
in a call into eglSwapBuffers which would attach a buffer to the surface
and commit. Unfortunately this would happen before the role for the
surface was set. This would result in the compositor not relaying to the
client that the desired frame was shown.
With this change the call to wl_shell_surface_set_toplevel is always
made before the first redraw.
https://bugzilla.gnome.org/show_bug.cgi?id=704457
2b8fcfd75b