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!

Towards possibly using Null pool for some nil objects.

Right now only Old Italic is marked as such.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1000

For consistency.