src/xdg-popup.c: choose output depending on xdg-positioner

src/xdg-popup.c: choose output depending on xdg-positioner

wlr_popup->current.geometry.{x,y} are usually zero on initial commit, so
xdg-popups were always unconstrained against the output which contains
the top-left of the parent toplevel.

This commit changes xdg-popups to be unconstrained against the output
which contains the top-left of preliminary popup geometry designated by
xdg-positioner given as an argument of "get_popup" or "reposition"
requests.