Multiface fonts supported in load but not range

Fonts with multiple faces are supported by looping over each face and accessing the number of faces once a valid FT_Face object is available. Like we do here:

- looping: https://github.com/mapbox/node-fontnik/blob/35feb88c121abbc6393f5236d7591035a1eb82cb/src/glyphs.cpp#L197
- accessing `ft_face->num_faces`: https://github.com/mapbox/node-fontnik/blob/35feb88c121abbc6393f5236d7591035a1eb82cb/src/glyphs.cpp#L205-L206

But the `num_faces = ft_face->num_faces;` is only done in `LoadAsync` and not `RangeAsync`

/cc @mikemorris

Start in src/glyphs.cpp by comparing LoadAsync, including its loop over faces and access to ft_face->num_faces, with RangeAsync. Update the range path so multiface fonts receive equivalent handling, then verify that range generation works across all faces.

Written by the indexing model from the issue text.