[arabic] Make ZWJ prevent ligatures instead of facilitating it

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.

src/hb-ot-layout-private.hh

diff --git a/src/hb-ot-layout-private.hh b/src/hb-ot-layout-private.hh
index 4e20a8f..3f02c80 100644
--- a/src/hb-ot-layout-private.hh
+++ b/src/hb-ot-layout-private.hh
@@ -107,6 +107,12 @@
   return !!(info->unicode_props0() & MASK0_ZWJ);
 }
 
+inline void
+_hb_glyph_info_flip_joiners (hb_glyph_info_t *info)
+{
+  info->unicode_props0() ^= MASK0_ZWNJ | MASK0_ZWJ;
+}
+
 
 #define hb_ot_layout_from_face(face) ((hb_ot_layout_t *) face->shaper_data.ot)
 

src/hb-ot-shape-complex-arabic.cc

diff --git a/src/hb-ot-shape-complex-arabic.cc b/src/hb-ot-shape-complex-arabic.cc
index a57e81a..d549945 100644
--- a/src/hb-ot-shape-complex-arabic.cc
+++ b/src/hb-ot-shape-complex-arabic.cc
@@ -157,6 +157,11 @@
 
 
 static void
+nuke_joiners (const hb_ot_shape_plan_t *plan,
+	      hb_font_t *font,
+	      hb_buffer_t *buffer);
+
+static void
 arabic_fallback_shape (const hb_ot_shape_plan_t *plan,
 		       hb_font_t *font,
 		       hb_buffer_t *buffer);
@@ -176,6 +181,8 @@
    * TODO: Add test cases for these two.
    */
 
+  map->add_gsub_pause (nuke_joiners);
+
   map->add_global_bool_feature (HB_TAG('c','c','m','p'));
   map->add_global_bool_feature (HB_TAG('l','o','c','l'));
 
@@ -314,6 +321,17 @@
 
 
 static void
+nuke_joiners (const hb_ot_shape_plan_t *plan HB_UNUSED,
+	      hb_font_t *font HB_UNUSED,
+	      hb_buffer_t *buffer)
+{
+  unsigned int count = buffer->len;
+  for (unsigned int i = 0; i < count; i++)
+    if (_hb_glyph_info_is_zwj (&buffer->info[i]))
+      _hb_glyph_info_flip_joiners (&buffer->info[i]);
+}
+
+static void
 arabic_fallback_shape (const hb_ot_shape_plan_t *plan,
 		       hb_font_t *font,
 		       hb_buffer_t *buffer)