Simplify ZWJ-skipping a bit

Towards disabling ZWJ-skipping in certain GPOS contexts.

Part of https://github.com/flutter/flutter/issues/16886

src/hb-ot-layout-gsubgpos.hh

diff --git a/src/hb-ot-layout-gsubgpos.hh b/src/hb-ot-layout-gsubgpos.hh
index 6ff19e2..031b39b 100644
--- a/src/hb-ot-layout-gsubgpos.hh
+++ b/src/hb-ot-layout-gsubgpos.hh
@@ -344,10 +344,10 @@
       match_glyph_data = nullptr;
       matcher.set_match_func (nullptr, nullptr);
       matcher.set_lookup_props (c->lookup_props);
-      /* Ignore ZWNJ if we are matching GSUB context, or matching GPOS. */
+      /* Ignore ZWNJ if we are matching GPOS, or matching GSUB context and asked to. */
       matcher.set_ignore_zwnj (c->table_index == 1 || (context_match && c->auto_zwnj));
-      /* Ignore ZWJ if we are matching GSUB context, or matching GPOS, or if asked to. */
-      matcher.set_ignore_zwj  (c->table_index == 1 || (context_match || c->auto_zwj));
+      /* Ignore ZWJ if we are matching context, or asked to. */
+      matcher.set_ignore_zwj  (context_match || c->auto_zwj);
       matcher.set_mask (context_match ? -1 : c->lookup_mask);
     }
     inline void set_lookup_props (unsigned int lookup_props)

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

diff --git a/src/hb-ot-shape-complex-indic.cc b/src/hb-ot-shape-complex-indic.cc
index 369078c..c1aa1d0 100644
--- a/src/hb-ot-shape-complex-indic.cc
+++ b/src/hb-ot-shape-complex-indic.cc
@@ -95,7 +95,8 @@
  * Indic shaper.
  */
 
-struct feature_list_t {
+struct feature_list_t
+{
   hb_tag_t tag;
   hb_ot_map_feature_flags_t flags;
 };
@@ -130,7 +131,10 @@
   {HB_TAG('b','l','w','s'), F_GLOBAL},
   {HB_TAG('p','s','t','s'), F_GLOBAL},
   {HB_TAG('h','a','l','n'), F_GLOBAL},
-  /* Positioning features, though we don't care about the types. */
+  /*
+   * Positioning features.
+   * We don't care about the types.
+   */
   {HB_TAG('d','i','s','t'), F_GLOBAL},
   {HB_TAG('a','b','v','m'), F_GLOBAL},
   {HB_TAG('b','l','w','m'), F_GLOBAL},
@@ -158,12 +162,14 @@
   _BLWS,
   _PSTS,
   _HALN,
+
   _DIST,
   _ABVM,
   _BLWM,
 
   INDIC_NUM_FEATURES,
-  INDIC_BASIC_FEATURES = INIT /* Don't forget to update this! */
+  INDIC_BASIC_FEATURES = INIT, /* Don't forget to update this! */
+  INDIC_SUBST_FEATURES = _DIST /* Don't forget to update this! */
 };
 
 static void
@@ -199,14 +205,19 @@
 
   unsigned int i = 0;
   map->add_gsub_pause (initial_reordering);
+
   for (; i < INDIC_BASIC_FEATURES; i++) {
     map->add_feature (indic_features[i].tag, 1, indic_features[i].flags | F_MANUAL_ZWJ | F_MANUAL_ZWNJ);
     map->add_gsub_pause (nullptr);
   }
+
   map->add_gsub_pause (final_reordering);
-  for (; i < INDIC_NUM_FEATURES; i++) {
+
+  for (; i < INDIC_SUBST_FEATURES; i++)
     map->add_feature (indic_features[i].tag, 1, indic_features[i].flags | F_MANUAL_ZWJ | F_MANUAL_ZWNJ);
-  }
+
+  for (; i < INDIC_NUM_FEATURES; i++)
+    map->add_feature (indic_features[i].tag, 1, indic_features[i].flags);
 
   map->add_global_bool_feature (HB_TAG('c','a','l','t'));
   map->add_global_bool_feature (HB_TAG('c','l','i','g'));

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

diff --git a/src/hb-ot-shape-complex-khmer.cc b/src/hb-ot-shape-complex-khmer.cc
index 0b5b50a..7a88aaa 100644
--- a/src/hb-ot-shape-complex-khmer.cc
+++ b/src/hb-ot-shape-complex-khmer.cc
@@ -32,7 +32,8 @@
  * Khmer shaper.
  */
 
-struct feature_list_t {
+struct feature_list_t
+{
   hb_tag_t tag;
   hb_ot_map_feature_flags_t flags;
 };
@@ -57,7 +58,10 @@
   {HB_TAG('a','b','v','s'), F_GLOBAL},
   {HB_TAG('b','l','w','s'), F_GLOBAL},
   {HB_TAG('p','s','t','s'), F_GLOBAL},
-  /* Positioning features, though we don't care about the types. */
+  /*
+   * Positioning features.
+   * We don't care about the types.
+   */
   {HB_TAG('d','i','s','t'), F_GLOBAL},
   {HB_TAG('a','b','v','m'), F_GLOBAL},
   {HB_TAG('b','l','w','m'), F_GLOBAL},
@@ -77,12 +81,14 @@
   _ABVS,
   _BLWS,
   _PSTS,
+
   _DIST,
   _ABVM,
   _BLWM,
 
   KHMER_NUM_FEATURES,
-  KHMER_BASIC_FEATURES = _PRES /* Don't forget to update this! */
+  KHMER_BASIC_FEATURES = _PRES, /* Don't forget to update this! */
+  KHMER_SUBST_FEATURES = _DIST, /* Don't forget to update this! */
 };
 
 static void
@@ -121,15 +127,16 @@
   map->add_global_bool_feature (HB_TAG('c','c','m','p'));
 
   unsigned int i = 0;
-  for (; i < KHMER_BASIC_FEATURES; i++) {
+  for (; i < KHMER_BASIC_FEATURES; i++)
     map->add_feature (khmer_features[i].tag, 1, khmer_features[i].flags | F_MANUAL_ZWJ | F_MANUAL_ZWNJ);
-  }
 
   map->add_gsub_pause (clear_syllables);
 
-  for (; i < KHMER_NUM_FEATURES; i++) {
+  for (; i < KHMER_SUBST_FEATURES; i++)
     map->add_feature (khmer_features[i].tag, 1, khmer_features[i].flags | F_MANUAL_ZWJ | F_MANUAL_ZWNJ);
-  }
+
+  for (; i < KHMER_NUM_FEATURES; i++)
+    map->add_feature (khmer_features[i].tag, 1, khmer_features[i].flags);
 
   map->add_global_bool_feature (HB_TAG('c','a','l','t'));
   map->add_global_bool_feature (HB_TAG('c','l','i','g'));

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

diff --git a/src/hb-ot-shape-complex-myanmar.cc b/src/hb-ot-shape-complex-myanmar.cc
index 8466f81..8069e3d 100644
--- a/src/hb-ot-shape-complex-myanmar.cc
+++ b/src/hb-ot-shape-complex-myanmar.cc
@@ -54,7 +54,14 @@
   HB_TAG('a','b','v','s'),
   HB_TAG('b','l','w','s'),
   HB_TAG('p','s','t','s'),
-  /* Positioning features, though we don't care about the types. */
+};
+static const hb_tag_t
+positioning_features[] =
+{
+  /*
+   * Positioning features.
+   * We don't care about the types.
+   */
   HB_TAG('d','i','s','t'),
   /* Pre-release version of Windows 8 Myanmar font had abvm,blwm
    * features.  The released Windows 8 version of the font (as well
@@ -96,14 +103,20 @@
 
 
   map->add_gsub_pause (initial_reordering);
+
   for (unsigned int i = 0; i < ARRAY_LENGTH (basic_features); i++)
   {
     map->add_feature (basic_features[i], 1, F_GLOBAL | F_MANUAL_ZWJ);
     map->add_gsub_pause (nullptr);
   }
+
   map->add_gsub_pause (final_reordering);
+
   for (unsigned int i = 0; i < ARRAY_LENGTH (other_features); i++)
     map->add_feature (other_features[i], 1, F_GLOBAL | F_MANUAL_ZWJ);
+
+  for (unsigned int i = 0; i < ARRAY_LENGTH (positioning_features); i++)
+    map->add_feature (positioning_features[i], 1, F_GLOBAL);
 }
 
 static void

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

diff --git a/src/hb-ot-shape-complex-use.cc b/src/hb-ot-shape-complex-use.cc
index 676b15b..62aef7e 100644
--- a/src/hb-ot-shape-complex-use.cc
+++ b/src/hb-ot-shape-complex-use.cc
@@ -86,7 +86,14 @@
   HB_TAG('h','a','l','n'),
   HB_TAG('p','r','e','s'),
   HB_TAG('p','s','t','s'),
-  /* Positioning features, though we don't care about the types. */
+};
+static const hb_tag_t
+positioning_features[] =
+{
+  /*
+   * Positioning features.
+   * We don't care about the types.
+   */
   HB_TAG('d','i','s','t'),
   HB_TAG('a','b','v','m'),
   HB_TAG('b','l','w','m'),
@@ -146,9 +153,13 @@
     map->add_feature (arabic_features[i], 1, F_NONE);
   map->add_gsub_pause (nullptr);
 
-  /* "Standard typographic presentation" and "Positional feature application" */
+  /* "Standard typographic presentation" */
   for (unsigned int i = 0; i < ARRAY_LENGTH (other_features); i++)
     map->add_feature (other_features[i], 1, F_GLOBAL | F_MANUAL_ZWJ);
+
+  /* "Positional feature application" */
+  for (unsigned int i = 0; i < ARRAY_LENGTH (positioning_features); i++)
+    map->add_feature (positioning_features[i], 1, F_GLOBAL);
 }
 
 struct use_shape_plan_t