Disable kern Format2.

Fix kerx Format2.  Manually tested this with Tamil MN font and it works:

$ HB_OPTIONS=aat ./hb-shape Tamil\ MN.ttc -u 0B94,0B95
[tgv_au=0+3435|tgc_ka=1@-75,0+1517]

 HB_OPTIONS=aat ./hb-shape Tamil\ MN.ttc -u 0B94,0B95 --features=-kern
[tgv_au=0+3510|tgc_ka=1+1592]

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

We never tested compiling without it.  Just kill it.  We always build
our own shaper.

This speeds up Roboto shaping by ~10%.  I was hoping for more.
Still, good defense against lookups with many subtables.

In preparation to move add per-subtable set digests...

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.

For 329f2401

max_ops is signed.

Since we consume it legitimately during shaping.

Still, not hooked.

Not hooked up to be called yet.

Makes our FT-backed hb_font_t safe to use from multiple threads.  Still,
the underlying FT_Face should NOT be used from other threads by client
or other libraries.

Maybe I add a lock()/unlock() public API ala PangoFT2 and cairo-ft.
Maybe not.

**Finally**!  Casual users can stop caring about font-funcs completely now,
like they haven't been needing to care re unicode-funcs for a few years.

Now that we have get_h_advances() and get_nominal_glyphs() implemented, the
overhead of doing a proper atomic load would be once per run, NOT once per
glyph.  So, no need to pre-load the tables to avoid that overhead.

As such, hb_ot_font_set_funcs() has become really cheap.  Can *finally* make
it be default font functions on all newly created fonts!