Allow disabling MS/Symbol cmap support in ot-font

[behdad]

See thread at:
https://lists.freedesktop.org/archives/harfbuzz/2018-January/006287.html

[behdad]

I like to add:

+          hb_face_set_symbol_mapping
+          hb_face_get_symbol_mapping

such that hb-ft, hb-ot-font, as well as custom impls can use it.

[behdad]

The symbol_mapping is just a bool to indicate whether mapping should be done for symbol fonts. It does NOT involve providing a custom mapping.

[behdad]

Or maybe API should be called "non_unicode" instead of "symbol", or "legacy". Also should be hb_ot_face_... not hb_face... Humm. Face doesn't have room to store that. So should be on font.

hb_ot_font_[sg]et_legacy_charmap? I don't like the name.

[behdad]

I lost a comment it seems. I said we might want to map non-Unicode Arabic in the same spot as well.

#681 (comment)