Based on patch from Jonathan Kew.

Needs more cleaning up and documentation.

New API:
hb_buffer_diff_flags_t
hb_buffer_diff()

Not all shapers code is updated to set this properly.
GSUB and Arabic shaper are updated.
GPOS and other shapers are NOT.

Fixes https://github.com/behdad/harfbuzz/issues/224

Ouch!

Fixes https://github.com/behdad/harfbuzz/issues/103

New API: HB_BUFFER_SERIALIZE_FLAG_GLYPH_EXTENTS

hb-shape now accepts --show-extents.

Patch from Simon Cozens.

Ie, don't call get_positions() if positions are not
requested for serialization.

Simplifies hb_in_range() calls as the type can be inferred.
The rest is obsessiveness, I admit.

Fixes https://github.com/behdad/harfbuzz/pull/17

Patch from Emil Eklund.

https://bugzilla.redhat.com/show_bug.cgi?id=1001645

Using a ragel machine.

Using a ragel machine.