[OpenJDK 2D-Dev] 7162125: [macosx] A font has different behaviour for ligatures depending on its creation mode

[Philip Race]

Bug: https://bugs.openjdk.java.net/browse/JDK-7162125
Fix: http://cr.openjdk.java.net/~prr/7162125/

The problem is that complex text layout and accompanying typographic support
is largely limping along on OS X and then only thanks to some canonical
tables that the ICU library has.

The bug is that the internal font "peer" that passes the font tables
containing the necessary information for OpenType (and AAT) layout
does not implement the necessary functions. So there is no way to
get the information that the font itself contains, resulting in incorrect
rendering.

The fix needs to be to add to the osx "CFont" class  the necessary
code to pass that information.  That is not so hard.

But there is an additional complication that the OS X code uses
"fallback" or "substitution" or "cascading" fonts and so a font
like "Lucida Grande" might report that it supports code points
from  Indic scripts when it does not.  It does this by reporting
negative glyph codes for such cases and fixes it up in the final
rendering step. The problem with that is that layout needs to
use the font-specific tables and this subverts that. We need
to know for sure which code points are supported and split
into runs of text that the font supports. So we need to find
the real fonts that the CFont uses. In core text this is obtainable
as the cascading font list. We can get this and construct a
CompositeFont and make its glyph mapper check that there
are no *negative* glyph codes not just no zero ones.

Now we can give layout proper runs and tables.
The final bit is that rendering needs to also know that if it
is passed a font that might implement FontSubstition that
it needs to expect that layout returned glyph codes that
are intended to be used with the Composite.

A reliable regression test is tricky. I have looked for a simple
code point sequence that an installed OS X font provides that I can
"measure" to see that the ligature forms but have not found one.

I have however tested using the font provided in
https://bugs.openjdk.java.net/browse/JDK-8139741
but I cannot check that in.

-phil.