wayland backend: fix xdg-shell for weston
wayland backend: fix xdg-shell for weston

 - xdg toplevel configure can be called with 0 width/height,
in that case we are free to do as we like (so do nothing)
 - need a display roundtrip after everything is setup but before
we start attaching buffers to the surface

ascent12/wlroots,ascent12/wlroots,swaywm/wlroots,SirCmpwn/wlroots,swaywm/wlroots