window-actor: Speed up mask creation

Due to a conditional error, meta_region_builder_add_rectangle was called
on every single blank pixel, rather than at the end of spans. With the new
rename, it's fairly clear to see the error. Fix the check to ensure that
we no longer make extraneous calls to meta_region_builder_add_rectangle.

https://bugzilla.gnome.org/show_bug.cgi?id=691874