Currently, this information is not used since cairo_show_glyphs
deals with color glyphs for us. But when we get to uploading
glyphs to a texture atlas, we will need it to do the right thing.

We don't look at individual glyphs here, but just whether the
font has the has-color flag set. In practice, all glyphs in
such a font will be color glyphs, and we can avoid loading all
the glyphs this way.