[subset] In LigatureSubst subsetting, check if the ligature glyph is in glyphset.

Otherwise coverage will not match the retained ligature sets.

src/OT/Layout/GSUB/Ligature.hh

@@ -29,6 +29,9 @@ struct Ligature
   bool intersects (const hb_set_t *glyphs) const
   { return hb_all (component, glyphs); }
 
+  bool intersects_lig_glyph (const hb_set_t *glyphs) const
+  { return glyphs->has(ligGlyph); }
+
   void closure (hb_closure_context_t *c) const
   {
     if (!intersects (c->glyphs)) return;

src/OT/Layout/GSUB/LigatureSet.hh

@@ -34,6 +34,18 @@ struct LigatureSet
     ;
   }
 
+  bool intersects_lig_glyph (const hb_set_t *glyphs) const
+  {
+    return
+    + hb_iter (ligature)
+    | hb_map (hb_add (this))
+    | hb_map ([glyphs] (const Ligature<Types> &_) { 
+      return _.intersects_lig_glyph (glyphs) && _.intersects (glyphs);
+    })
+    | hb_any
+    ;
+  }
+
   void closure (hb_closure_context_t *c) const
   {
     + hb_iter (ligature)

src/OT/Layout/GSUB/LigatureSubstFormat1.hh

@@ -130,7 +130,7 @@ struct LigatureSubstFormat1_2
     + hb_zip (this+coverage, hb_iter (ligatureSet) | hb_map (hb_add (this)))
     | hb_filter (glyphset, hb_first)
     | hb_filter ([&] (const LigatureSet<Types>& _) {
-      return _.intersects (&glyphset);
+      return _.intersects_lig_glyph (&glyphset);
     }, hb_second)
     | hb_map (hb_first)
     | hb_sink (new_coverage);