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

Fixes https://github.com/behdad/harfbuzz/issues/223

Right now we cannot test this because it has to be tested using hb-fuzzer.
We should move all fuzzing tests from test/shaping/tests/fuzzed.tests to
test/fuzzing/ and have its own test runner.  At that point, should add
test from this issue as well.

Fixes https://github.com/behdad/harfbuzz/issues/227

Fixes https://github.com/behdad/harfbuzz/issues/169

This is what Microsoft's implementation does.  Marks that need advance
need to add it back using 'dist' or other feature in GPOS.  Update tests to
match.

Fixes https://github.com/behdad/harfbuzz/issues/211

What happens in that bug is that a mark is attached to base first,
then a second mark is cursive-chained to the first mark.  This only
"works" because it's in the Indic shaper where mark advances are
not zeroed.

Before, we didn't allow cursive to run on marks at all.  Fix that.
We also where updating mark major offsets at the end of GPOS, such
that changes in advance of base will not change the mark attachment
position.  That was superior to the alternative (which is what Uniscribe
does BTW), but made it hard to apply cursive to the mark after it
was positioned.  We could track major-direction offset changes and
apply that to cursive in the post process, but that's a much trickier
thing to do than the fix here, which is to immediately apply the
major-direction advance-width offsets...  Ie.:

https://github.com/behdad/harfbuzz/issues/211#issuecomment-183194739

If this breaks any fonts, the font should be fixed to do mark attachment
after all the advances are set up first (kerning, etc).

Finally, this, still doesn't make us match Uniscribe, for I explained
in that bug.  Looks like Uniscribe applies minor-direction cursive
adjustment immediate as well.  We don't, and we like it our way, at
least for now.  Eg. the sequence in the test case does this:

- The first subscript attaches with mark-to-base, moving in x only,
- The second subscript attaches with cursive attachment to first subscript
  moving in x only,
- A final context rule moves the first subscript up by 104 units.

The way we do, the final shift-up, also shifts up the second subscript
mark because it's cursively-attached.  Uniscribe doesn't.  We get:

[ttaorya=0+1307|casubscriptorya=0@-242,104+-231|casubscriptnarroworya=0@20,104+507]

while Uniscribe gets:

[ttaorya=0+1307|casubscriptorya=0@-242,104+-211|casubscriptnarroworya=0+487]

note the different y-offset of the last glyph.  In our view, after cursive,
things move together, period.

No effect.

Part of fixing https://github.com/behdad/harfbuzz/issues/211

Right now the position_finish_advances() is empty.  To be used for
spacing attachments proposal later.

Will do nothing.  Just useful for merging two functions.

Differentiate, using new attach_type().

No behavior change.  Preparing to unify how cursive and mark attachments
work.

Is confusing.  I already hit it myself.  Remove.  We can optimize
ASCII based on Unicode properties.  But should not do based on
assumptions on the font.

See thread at:
https://lists.freedesktop.org/archives/harfbuzz/2016-February/005462.html

This adds to the autotools build system so that the (experimental)
DirectWrite support for HarfBuzz is built (and dist'ed).

This moves all the source listings in src/Makefile.am,
src/hb-ucdn/Makefile.am and util/Makefile.am into separate Makefile
snippets, so that they may be shared between different Makefile-based
build systems, such as NMake for Visual Studio.

Hopefully
fixes https://github.com/behdad/harfbuzz/issues/214

Fixes https://github.com/behdad/harfbuzz/issues/212

This speeds up shaping the Amiri font by over 15%.

This was primarily needed for my work on OpenType GX, since
we will be collecting only sublookups that are "active" for
current font instance; but it's a nice boost in general as
well.

We might, in the future, collect subtables in the lookup_accel.
That would also allow us to do a per-subtbale set-digest, which
should speed things up some more, specially for ContextChainFormat3
lookups...  Amiri, for example, contains one lookup with 53
subtables!

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

Might add italic-angle, underline/strikethrough-position/thickness
in the future.  Do this before new struct goes into a release.

This happens with at least one test font I have.

See previous commit.

The font Garamond Premier Pro Caption (and possibly many other
Adobe fonts), have many FeatureParamsSize tables with the old
wrong offset.  We handle fixing those up, but they were still
contributing to edit_count, and when I reduced HB_SANITIZE_MAX_EDIT
from 100 to 8 in 14c2de32, these
fonts were now getting GPOS dropped and hence kerning disabled.

Fix, by not counting edits made towareds offset fix-up.  I'll
also increase edit count again, in the next commit.