Works beautifully!  Test coming.

Fixes https://github.com/harfbuzz/harfbuzz/issues/322

Oops.  Was checking for kern feature in GSUB, not GPOS.

Hopefully more clear.

Previously we only did if there was no GPOS whatsoever.  This applies
to Arial, Times New Roman, etc in Win7.  Was not kerning before.  It
is now.

Not hooked up to be called yet.

Fixes https://github.com/harfbuzz/harfbuzz/issues/250

We use scratch-flags to short-circuit this function.  No need for previous
early loop.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1216

New API:
hb_buffer_set_invisible_codepoint()
hb_buffer_get_invisible_codepoint()

hb-shape / hb-view --invisible-codepoint

https://github.com/harfbuzz/harfbuzz/commit/a01194aaf4c15160330b4042066263b2c963b658#commitcomment-30772041

"The tag space of tags consisting of four uppercase letters (A-Z) with no punctuation,
spaces, or numbers, is reserved as a vendor space. Font vendors may use such tags to
identify private features."

One early, before script-specific features, one late, after.  Allows font
developers to detect us and behave differently if needed.

Also, don't attach ZWNJ to previous cluster.  Closer to Unicode graphemes.

This adds a new grapheme bit.  Not used yet.

Part of https://github.com/harfbuzz/harfbuzz/issues/1159

Fixes https://github.com/harfbuzz/harfbuzz/issues/1189

Use rand=255 to mean "randomize".

Part of https://github.com/harfbuzz/harfbuzz/pull/803

Randomization only happens by default. If the user specifies a value for
'rand', that value is respected.

Sorry for the noise, downstream custom builders.  Please adjust.