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.

New API:
+hb_font_funcs_set_glyph_h_advances_func
+hb_font_funcs_set_glyph_v_advances_func
+hb_font_get_glyph_h_advances
+hb_font_get_glyph_h_advances_func_t
+hb_font_get_glyph_v_advances
+hb_font_get_glyph_v_advances_func_t

Also, uncomment code again, just "if (0)" it out, so it doesn't get stale again.

Ouch!