Does page lookup as needed.

The coverage already covered that.

The three "XXXXX"'s should be switched to false.  Doing that separately for ease
of bisecting...

Not optimized to use sortedness yet.  Also start putting in place infra
to faster reject bad data.

A version of Chandas.ttf found on some Chrome bots has 660kb of GPOS,
mostly junk.  That is causing 48 million of set->add() calls in
collect_glyphs(), which is insane.

In the upcoming commits, I'll be speeding that up by optimizing
add_sorted_array(), while also reducing work by rejecting out-of-sort
arrays quickly and propagate the rejection.

Part of https://bugs.chromium.org/p/chromium/issues/detail?id=794896

Now that pagesize is 8192, this feels better.

Fixes perf regression on some heavy fonts in Chrome's FT+HB
interaction.

See:
https://bugs.chromium.org/p/chromium/issues/detail?id=782220

More work to be done:
https://bugs.chromium.org/p/chromium/issues/detail?id=794896

To be used to optimize adding a whole bunch of (sorted) items at the same time,
as in CoverageFormat1.

Use a matrix to avoid a gazillion if's.

Fixes https://github.com/harfbuzz/harfbuzz/issues/640

Fixes https://github.com/harfbuzz/harfbuzz/issues/635

FT_Set_Var_Blend_Coordinates() is not available when "Multiple Master
font interface" is disabled in freetype's modules.cfg

Ouch!

Add tests.

Fixes https://github.com/harfbuzz/harfbuzz/issues/628

New API:
hb_coretext_font_create()

It's as good as it gets, and seems to be on par with previous set implementation
in my benchmark.

Would be great if someone can double-check my bitops.

With new set implementation, this became really costy.  Optimize it.
There's more to be done, but this shaves off most of the fat.

Part of fixing https://bugs.chromium.org/p/chromium/issues/detail?id=782220

From https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=4273

Particularly hazardous if the second layer mixes forward and backward lookups.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=4336

Related
https://github.com/harfbuzz/harfbuzz/issues/345
https://github.com/harfbuzz/harfbuzz/pull/629

SVG is unused. Just keeping it here.