[API] Add hb_buffer_[sg]et_not_found_glyph() and --not-found-glyph

Instead of using gid=0 when a character is not found in the font,
client can now set a custom value.  This is useful for shaper-driven
font fallback and to differentiate that from .notdef glyph.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1360
8 files changed