gtk-screen: Expose handler must not layout-line! during redraw-line!.

gtk-screen: Expose handler must not layout-line! during redraw-line!.

Renamed %layout-line! to emphasize that it should not be used without
interlocking with the expose handler, i.e. without without-interrupts.
Most calls are serialized by edwin-thread but the expose handler can
intervene (and likely WILL when running interpreted s-code), so all
calls should be without-interrupts, for now.

Punt exposing uninitialized lines.

Simplify the destruction of override text (a simple-text-ink) AND
ensure that it does not call %layout-line! via text-ink-pango-layout.