Enable applying GSUB/GPOS features in multiple segments

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=644184
among others.

Shapers now can request segmented feature application by calling
add_gsub_pause() or add_gpos_pause().  They can also provide a
callback to be called at the pause.  Currently the Arabic shaper
uses pauses to enforce certain feature application.  The Indic
shaper can use the same facility to pause and do reordering in the
callback.

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 edcd7fb..4cbdde1 100644
--- a/src/hb-ot-shape-complex-arabic.cc
+++ b/src/hb-ot-shape-complex-arabic.cc
@@ -153,12 +153,32 @@
 void
 _hb_ot_shape_complex_collect_features_arabic (hb_ot_shape_planner_t *planner, const hb_segment_properties_t  *props)
 {
-  /* ArabicOT spec enables 'cswh' for Arabic where as for basic shaper it's disabled by default. */
-  planner->map.add_bool_feature (HB_TAG('c','s','w','h'));
+  /* For Language forms (in ArabicOT speak), we do the iso/fina/medi/init together,
+   * then rlig and calt each in their own stage.  This makes IranNastaliq's ALLAH
+   * ligature work correctly. It's unfortunate though...
+   *
+   * This also makes Arial Bold in Windows7 work.  See:
+   * https://bugzilla.mozilla.org/show_bug.cgi?id=644184
+   *
+   * TODO: Add test cases for these two.
+   */
+
+  planner->map.add_gsub_pause (NULL, NULL);
 
   unsigned int num_features = props->script == HB_SCRIPT_SYRIAC ? SYRIAC_NUM_FEATURES : COMMON_NUM_FEATURES;
   for (unsigned int i = 0; i < num_features; i++)
     planner->map.add_bool_feature (arabic_syriac_features[i], false);
+
+  planner->map.add_gsub_pause (NULL, NULL);
+
+  planner->map.add_bool_feature (HB_TAG('r','l','i','g'));
+  planner->map.add_gsub_pause (NULL, NULL);
+
+  planner->map.add_bool_feature (HB_TAG('c','a','l','t'));
+  planner->map.add_gsub_pause (NULL, NULL);
+
+  /* ArabicOT spec enables 'cswh' for Arabic where as for basic shaper it's disabled by default. */
+  planner->map.add_bool_feature (HB_TAG('c','s','w','h'));
 }
 
 void