Ignore wl_output globals not bound by us

Gdk doesn't know the scale of output globals it didn't bind. This keeps them from entering the output list and triggering erroneous changes in surface scales.

Without !3898 (closed), Firefox must wait for the WITHDRAWN state to be removed before it can do anything with the popup, but in the current status quo there is an infinite loop in gdk that prevents the WITHDRAWN state from ever being unset, so another change is needed to prevent that.

@emersion this would be a partial fix for sway#6426 but not sway#6147.

cc @jadahl