[arabic] Implement Windows-1256 private shaping

Bug 1045139 - The Arabic text with "MS Sans Serif" font is rendered bad
https://bugzilla.mozilla.org/show_bug.cgi?id=1045139

This is only enabled on Windows platforms, and requires support from
Uniscribe to work.  But for clients that do hook up to Uniscribe, this
fixes shaping of Windows-1256-encoded bitmap fonts like "MS Sans Serif".

The code and table together have just less than a 1kb footprint when
enabled.

UNTESTED.  I might even have broken regular Arabic fallback shaping.
diff --git a/src/Makefile.am b/src/Makefile.am
index acedaa0..ba07ac5 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -91,6 +91,7 @@
 	hb-ot-shape-complex-arabic.cc \
 	hb-ot-shape-complex-arabic-fallback.hh \
 	hb-ot-shape-complex-arabic-table.hh \
+	hb-ot-shape-complex-arabic-win1256.hh \
 	hb-ot-shape-complex-default.cc \
 	hb-ot-shape-complex-hangul.cc \
 	hb-ot-shape-complex-hebrew.cc \