Was broken in 615d00ea.

Fixes https://github.com/behdad/harfbuzz/pull/48

Micro-test added.

Previously, we expected users to provide BOT/EOT flags when the
text *segment* was at paragraph boundaries.  This meant that for
clients that provide full paragraph to HarfBuzz (eg. Pango), they
had code like this:

  hb_buffer_set_flags (hb_buffer,
                       (item_offset == 0 ? HB_BUFFER_FLAG_BOT : 0) |
                       (item_offset + item_length == paragraph_length ?
                        HB_BUFFER_FLAG_EOT : 0));

  hb_buffer_add_utf8 (hb_buffer,
                      paragraph_text, paragraph_length,
                      item_offset, item_length);

After this change such clients can simply say:

  hb_buffer_set_flags (hb_buffer,
                       HB_BUFFER_FLAG_BOT | HB_BUFFER_FLAG_EOT);

  hb_buffer_add_utf8 (hb_buffer,
                      paragraph_text, paragraph_length,
                      item_offset, item_length);

Ie, HarfBuzz itself checks whether the segment is at the beginning/end
of the paragraph.  Clients that only pass item-at-a-time to HarfBuzz
continue not setting any flags whatsoever.

Another way to put it is: if there's pre-context text in the buffer,
HarfBuzz ignores the BOT flag.  If there's post-context, it ignores
EOT flag.

Ouch!

Follows the order of the Arabic/Syriac specs.  Also don't stop
between rlig and calt in non-Arabic scripts.

Micro-tests for Arabic and Mongolian added for the latter.

This reverts bf029281 and fixes it properly.  That commit
was not enough as it was only inheriting the shaping_action
for prev_action, but not curr_action.

Micro-test added.

https://code.google.com/p/chromium/issues/detail?id=393896

Old table was from 6.2.  Remove hard-coded Mongolian and Phags-pa data.
This completes support for new scripts Manichian and Psaltar Pahlavi.

No functional change.

I believe Windows 8 disables it, and spec update dated
Jan 2014 also clearly says it's disabled by default:

  http://www.microsoft.com/typography/OpenTypeDev/arabic/intro.htm#features

No shaper has more than one behavior re this, so no need for a callback.

This reverts commit d5bd0590.

The reasoning behind that logic was flawed and made under
a misunderstanding of the original problem, and caused
regressions as reported by Jonathan Kew in thread titled
"tibetan marks" in Oct 2013.  Apparently I have had fixed
the original problem with this commit:

  7e08f125

So, revert the faulty commit and everything seems to be in good
shape.

See:
http://lists.freedesktop.org/archives/harfbuzz/2013-April/003101.html

Unicode 6.2.0 Section 16.2 / Figure 16.3 says:

"For backward compatibility, between Arabic characters a ZWJ acts just
like the sequence <ZWJ, ZWNJ, ZWJ>, preventing a ligature from forming
instead of requesting the use of a ligature that would not normally be
used. As a result, there is no plain text mechanism for requesting the
use of a ligature in Arabic text."

As such, we flip internal zwj to zwnj flags for GSUB matching, which
means it will block ligation in all features, unless the font
explicitly matches U+200D glyph.  This doesn't affect joining behavior.

Windows 8 doesn't, and the spec will be fixed.

Mozilla Bug 873902 - Display Arabic text with diacritics is bad
https://bugzilla.mozilla.org/show_bug.cgi?id=873902

Testing shows that this is closer to what Uniscribe does.

Reported by Khaled Hosny:

"""
commit 56800027
...
This commit is causing a regression with Amiri, the string “هَٰذ” with
Uniscribe and HarfBuzz before this commit, gives:

	[uni0630.fina=3+965|uni0670.medi=0+600|uni064E=0@-256,0+0|uni0647.init=0+926]

But now it gives:

	[uni0630.fina=3+965|uni0670.medi=0+0|uni064E=0@-256,0+0|uni0647.init=0+926]

i.e. uni0670.medi is zeroed though it has a base glyph GDEF class.
"""

The test case is U+0647,U+064E,U+0670,U+0630 with Amiri.

This is the first character in Unicode to have Arabic left-joining
behavior.  Update the machine to recognize that.

Test case: U+A840,U+A872,U+A840.

Code cleanup.  No (intended) functional change.

Before, we were zeroing advance width of attached marks for
non-Indic scripts, and not doing it for Indic.

We have now three different behaviors, which seem to better
reflect what Uniscribe is doing:

  - For Indic, no explicit zeroing happens whatsoever, which
    is the same as before,

  - For Myanmar, zero advance width of glyphs marked as marks
    *in GDEF*, and do that *before* applying GPOS.  This seems
    to be what the new Win8 Myanmar shaper does,

  - For everything else, zero advance width of glyphs that are
    from General_Category=Mn Unicode characters, and do so
    before applying GPOS.  This seems to be what Uniscribe does
    for Latin at least.

With these changes, positioning of all tests matches for Myanmar,
except for the glitch in Uniscribe not applying 'mark'.  See preivous
commit.

Ouch!

New API:

	hb_buffer_flags_t

	HB_BUFFER_FLAGS_DEFAULT
	HB_BUFFER_FLAG_BOT
	HB_BUFFER_FLAG_EOT
	HB_BUFFER_FLAG_PRESERVE_DEFAULT_IGNORABLES

	hb_buffer_set_flags()
	hb_buffer_get_flags()

We use the BOT flag to decide whether to insert dottedcircle if the
first char in the buffer is a combining mark.

The PRESERVE_DEFAULT_IGNORABLES flag prevents removal of characters like
ZWNJ/ZWJ/...

Had to do some refactoring to make this happen...

Under uniscribe bug compatibility mode, we still plit them
Uniscrie-style, but Jonathan and I convinced ourselves that there is no
harm doing this the Unicode way.  This change makes that happen, and
unbreaks free Sinhala fonts.

For U+1880..U+1886 Uniscribe thinks they are non-joining.
For U+1887 Uniscribe thinks it's joining, but looks wrong to me.
For now, match Uniscribe.

Through the Arabic shaper.  It's similar to Mongolian.

That's what the spec says, and what Uniscribe does.

Now we respect Arabic joining across runs.

All of init/medi/fina/isol and rlig implemented.

Let there be dragons... ⻯

This will eventually allow us to skip marks, as well as (fallback)
attach marks to ligature components of fallback-shaped Arabic.
That would be pretty cool.  I kludged GDEF props in, so mark-skipping
works, but the produced ligature id/components will be cleared later
by substitute_start() et al.

Perhaps using a synthetic table for Arabic fallback shaping was a better
idea.  The current approach has way too many layering violations...

This reverts commit 3e0a0397.

I know remember why that line is there :).