cogl-pango: Special case alpha of 0

pango_renderer_get_alpha() returns 0 to indicate that the alpha value
should be inherited from the environment, but we are passing it on
(and therefore making the text fully translucent).

Instead, make the text fully opaque as expected.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1156

cogl/cogl-pango/cogl-pango-render.c

@@ -730,7 +730,7 @@ cogl_pango_renderer_set_color_for_part (PangoRenderer   *renderer,
                                 pango_color->red >> 8,
                                 pango_color->green >> 8,
                                 pango_color->blue >> 8,
-                                alpha >> 8);
+                                alpha ? alpha >> 8 : 0xff);
 
       _cogl_pango_display_list_set_color_override (priv->display_list, &color);
     }