Based on patch from Jonathan Kew.

Needs more cleaning up and documentation.

New API:
hb_buffer_diff_flags_t
hb_buffer_diff()

We break and shape fragments and reconstruct shape result from them.
Remains to compare to original buffer.  Going to add some buffer
comparison API and use here, instead of open-coding.

Ouch!

Makes consumption easier.

We were relying on cluster merges not requiring unsafe flagging because
they get merged.  If cluster level requests no merging, then we flag
unsafe when merge would have happened.

Right now it checks for monotone cluster values.  Other checks to be added.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=3011

This shouldn't matter.

Always use the algorithm for non-monotone case.  It's more robust.

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

Part of https://github.com/behdad/harfbuzz/issues/509

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=1452#c5

CC https://github.com/behdad/harfbuzz/issues/139

1. Handle segment with two entries correctly,

2. Fix rounding math.  Ouch!

Fixes https://github.com/behdad/harfbuzz/issues/521

Fixes https://github.com/behdad/harfbuzz/issues/510

https://github.com/behdad/harfbuzz/issues/506

Before, that was printed using --debug (and in both hb-shape and hb-view).
Changed it, now hb-shape has a new command-line argument called --show-messages.
When invoked, it also respects other output formatting options.  The messages
are better formatted and printed to te same place that hb-shape output is
directed to.  Previously they were written to stderr.

Fixes https://github.com/behdad/harfbuzz/issues/506