Fixes https://github.com/behdad/harfbuzz/issues/615

We'll see which bots I broke.

Part of fixing https://github.com/behdad/harfbuzz/pull/605

https://travis-ci.org/behdad/harfbuzz/jobs/295039536#L3468
https://ci.appveyor.com/project/behdad/harfbuzz/build/1.0.697/job/dr1ujlntxafeqbej#L142

error: static_assert expression is not an integral constant expression

CoreText "point"s are not typographic points, but CSS pixels.  Ie.
they are 96 per inch, not 72 per inch.

Another attempt at fully fixing
https://github.com/behdad/harfbuzz/issues/360

Fixes bots.

Link libharfbuzz.so with -lm.  Surprising that we survived without
it so far!

Fixes https://github.com/behdad/harfbuzz/issues/474

SFNSText has kerx table which apparently is applied.

Attempt at fixing #360

Towards implementing optical sizing.  Untested; won't compile.

https://github.com/behdad/harfbuzz/issues/360

* Minor

* [coretext] Fix leak

* [coretext] Do not reset num_features

* [coretext] allow to disable kern; re-enabling doesn't seem to be working

Shape-plan caching is not implemented.

This seems to fix crash issues on 10.9.5 reported on Chrome, compare
crbug.com/549610

This one:

  map->mask = (1 << (next_bit + bits_needed)) - (1 << next_bit);

before the fix, the shift was done as an int, causing overflow
if it ever got to 1 << 31.  Sprinkle 'u's around.

Fixes https://bugs.chromium.org/p/chromium/issues/detail?id=634805

Fixes https://github.com/behdad/harfbuzz/issues/297

Fixes https://github.com/behdad/harfbuzz/issues/267