compositor.c: xdg-surface layout position
compositor.c: xdg-surface layout position

Add data structures to compositor.c xdg-surface implementation that allows them
to be rendered in the correct position in the layout, spanning outputs if
necessary.

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