[face] Expose cmap mapping

[behdad]

We have hb_face_collect_unicodes, we should expose the mapping counterpart the returns both a set and a map so people can walk the set and map to glyph.

[matthiasclasen]

I would expect something like

hb_bool_t
hb_face_get_glyph (hb_face_t      *face,
                   hb_codepoint_t  unicode,
                   hb_codepoint_t  variation_selector,
                   hb_codepoint_t *glyph)

?

[behdad]

hb_bool_t
hb_face_get_glyph (hb_face_t *face,
hb_codepoint_t unicode,
hb_codepoint_t variation_selector,
hb_codepoint_t *glyph)

Given that one can just create a font and get that already, I'm not very keen on adding it. Exposing cmap is for performance reasons to avoid having to call get_glyph on every glyph. I don't remember where it was needed that I filed this though.

[behdad]

both a set and a map so people can walk the set and map to glyph.

I added iterator to hb_map_t now. So the hb_set_t to such new API should be made optional.