Fixes 10% of the failures against all of Hindi Wikipedia!

Accepts --format=html now.

Doesn't do --color anymore.  That will go into a new hb-diff-colorize
tool.

As reported by Jonathan Kew on the list.

Experimental API for now.

U+0985 BENGALI LETTER A followed by U+09D7 BENGALI AU LENGTH MARK.
According to Bobby de Vos on the mailing list, this results in a dotted
circle with most shaping engines, but is a legitimate sequence in this
minority language.

We reached the consensus on the list to NOT implement dotted-circle
in HarfBuzz.

TODO: Tests for hb_set_t.

The GSUBGPOS part still missing.

Filling in.

Should have no visible effect.

To be used in the Indic shaper later.  Unused for now.