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.

New API:
HB_BUFFER_FLAG_REMOVE_DEFAULT_IGNORABLES
hb-shape / hb-view --remove-default-ignorables

One more text-rendering-tests test passing.  Eleven failing.

So I can merge this to master...

Also makes hb-ot-shape call morx for now instead of GSUB... Just for testing.

This only counts recursions right now.  Good start.

Hopefully...
Fixes https://github.com/behdad/harfbuzz/issues/429

This should affect mark attachment in vertical text.  I have no font to test,
but this sounds the right order.

Noticed while debugging https://github.com/behdad/harfbuzz/issues/532

Fixes tests/shaping/tests/cluster.tests

Fixes tests/automatic-fractions.tests

Makes consumption easier.

The numbers for right-to-left scripts are processed also from right to
left, so the order of applying “numr” and “dnom” features should be
reversed in such case.

Fixes https://github.com/behdad/harfbuzz/issues/395

New approach to fix this:
https://github.com/behdad/harfbuzz/commit/69f9fbc4200442a35484d3c790ae8f4979be5d60

Previous approach was reverted as it was too broad.  See context:
https://github.com/behdad/harfbuzz/issues/347#issuecomment-267838368

With U+05E9,U+05B8,U+05C1,U+05DC and Arial Unicode, we now (correctly) disable
GDEF and GPOS, so we get results very close to Uniscribe, but slightly different
since our fallback position logic is not exactly the same:

Before:		[gid1166=3+991|gid1142=0+737|gid5798=0+1434]
After:		[gid1166=3+991|gid1142=0@402,-26+0|gid5798=0+1434]
Uniscribe:	[gid1166=3+991|gid1142=0@348,0+0|gid5798=0+1434]

This reverts commit 69f9fbc4.

See https://github.com/behdad/harfbuzz/issues/347#issuecomment-268873401
Fixes https://github.com/behdad/harfbuzz/issues/347

Shape-plan caching is not implemented.