Fixes https://github.com/libass/libass/issues/212

Fixes https://code.google.com/p/chromium/issues/detail?id=547912

The coretext_aat shaper delegates to the regular coretext_..._ensure() functions, so coretext_aat_..._ensure() functions defined by these macros are unused. The compiler warns about them, which in turn can confuse people to think that the coretext_aat_..._ensure() functions weren't called by accident.

Fixes a -Wunused-label warning when building harfbuzz with clang -Wall.

Fixes https://github.com/behdad/harfbuzz/issues/103

Shouldn't cause *any* functional changes, but is more correct.

http://crbug.com/419769

Fixes https://code.google.com/p/chromium/issues/detail?id=476913

As discovered on Chrome Mac:
https://code.google.com/p/chromium/issues/detail?id=452326

As discovered on Chrome Mac:
https://code.google.com/p/chromium/issues/detail?id=452326

This was originally broken in:

	commit 5a0eed3b
	Author: Behdad Esfahbod <behdad@behdad.org>
	Date:   Mon Aug 11 23:47:16 2014 -0400

	    [coretext] Implement vertical shaping

Before we were not accounting for possible differences in x_scale and
y_scale, as well as the signs of those.  All should be in good shape
now.

Was very broken.  Now fixed and tested.

See comments.

Fixes vertical text.  CoreText backend is in very good shape now!

Also see:
5a0eed3b
25f4fb9b

Fixes http://github.com/behdad/harfbuzz/pull/36

Currently doesn't work though, we detect font fallback.  Apparently
matching on ct_font is not safe for this.  Looks like commit
25f4fb9b wasn't enough after all.

Wasn't needed after a6b8dc87.

Assert that all runs had expected direction, and take hint for
non-monotone clusters.

Have to use a CTTypesetter for this.

It's hard to handle all possible NULL returns from CoreText.  Add one
more...

Fixes https://githu.com/behdad/harfbuzz/issues/49
to the extent that it can be fixed.

Before, this shaper was returning UTF-16 cluster indices instead of
returning whatever cluster values the user had had set up in the buffer.
Ouch!

We can't really resize buffer and continue in this shaper as we are
using the scratch buffer for string_ref and log_cluster.  Restructure
shaper to retry from (almost) scratch.