* hb_ot_color_has_svg
* hb_ot_color_glyph_svg_create_blob

Now to hb_color_t.

COLR table has one function: return layers for a glyph, and we expose exactly
that, so should just wire it through.  Also use sub_array() for verifiable
safety.

Also, BaseGlyphRecord's null object is enough.  We don't need to special-case
the not-found.

We never return parallel arrays from functions.  That's inconvenient
and hard to bind.

Implement has_data() for realz.

We don't expose "gid" in API.

Sorry for the noise, downstream custom builders.  Please adjust.

The name id 0 is used as Copyright notice. It's quite unlikely that a
font supplies a color palette with the exact same name as the font's
copyright notice, but the API should not prevent this.

Also, try to fix a problem with GObject introspection, where the
auto-generated Python bindings could not return palette colors.