Fix compositing non-color glyphs in one case

The intention here is: "if show_text_glyphs is unsupported, try show_gyphs". But unsupported is not indicated by the status here, but by show_text_glyphs == NULL. The status does not change and is still whatever composite_color_glyphs set it to.

This showed up in https://github.com/harfbuzz/harfbuzz/issues/2675