[arabic] Do the joiner-flipping only for rlig feature

See comment.

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

@@ -159,7 +159,7 @@ static const struct arabic_state_table_entry {
 
 
 static void
-nuke_joiners (const hb_ot_shape_plan_t *plan,
+flip_joiners (const hb_ot_shape_plan_t *plan,
 	      hb_font_t *font,
 	      hb_buffer_t *buffer);
 
@@ -200,7 +200,6 @@ collect_features_arabic (hb_ot_shape_planner_t *plan)
    * work correctly.  See https://github.com/harfbuzz/harfbuzz/issues/505
    */
 
-  map->add_gsub_pause (nuke_joiners);
 
   map->enable_feature (HB_TAG('s','t','c','h'));
   map->add_gsub_pause (record_stch);
@@ -217,14 +216,29 @@ collect_features_arabic (hb_ot_shape_planner_t *plan)
     map->add_gsub_pause (nullptr);
   }
 
+  /* Normally, Unicode says a ZWNJ means "don't ligate".  In Arabic script
+   * however, it says a ZWJ should also mean "don't ligate".  So we convert
+   * a ZWJ to a ZWNJ for GSUB.  We want to revert it back to ZWJ before
+   * GPOS processing though.  So we just flip their roles, and flip back
+   * later.  Note that this makes a ZWNJ into ZWJ for GSUB stage, which
+   * means it would *not* break ligatures.  But since ligatures around
+   * ZWNJ are rare, we don't care.
+   *
+   * Since we don't currently have a way to apply a pause before GPOS
+   * starts, let's just do this dance around a few required GUSB features. */
+  map->add_gsub_pause (flip_joiners);
+
   map->add_feature (HB_TAG('r','l','i','g'), F_GLOBAL | F_HAS_FALLBACK);
+
   if (plan->props.script == HB_SCRIPT_ARABIC)
     map->add_gsub_pause (arabic_fallback_shape);
 
   /* No pause after rclt.  See 98460779bae19e4d64d29461ff154b3527bf8420. */
   map->enable_feature (HB_TAG('r','c','l','t'));
   map->enable_feature (HB_TAG('c','a','l','t'));
-  map->add_gsub_pause (nullptr);
+
+  /* And undo here. */
+  map->add_gsub_pause (flip_joiners);
 
   /* The spec includes 'cswh'.  Earlier versions of Windows
    * used to enable this by default, but testing suggests
@@ -381,14 +395,14 @@ setup_masks_arabic (const hb_ot_shape_plan_t *plan,
 
 
 static void
-nuke_joiners (const hb_ot_shape_plan_t *plan HB_UNUSED,
+flip_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;
   hb_glyph_info_t *info = buffer->info;
   for (unsigned int i = 0; i < count; i++)
-    if (_hb_glyph_info_is_zwj (&info[i]))
+    if (_hb_glyph_info_is_joiner (&info[i]))
       _hb_glyph_info_flip_joiners (&info[i]);
 }