Azure should re-use CGFont stored by QuartzFontHandle

[burg]

When making an azure scaled font, we currently use a CTFont wrapper to make a new CGFont instance. It would be better to re-use servo's existing CGFont reference. We'd have to add a trait to prevent circular dependencies, i.e.:

trait CGFontProvider {
   pure fn get_CGFont_ref() -> CGFontRef;
}

And then pass the font handle (as the trait type) to ScaledFont::new() instead of CTFont.