[instance] drop and don't collect lookups from feature variations when fully instancing.

This previously incorrectly collected lookups that could be reached via feature variations that are dropped and not activated by the current instance position.

src/hb-ot-layout-gsubgpos.hh

@@ -4142,8 +4142,11 @@ struct GSUBGPOSVersion1_2
   bool subset (hb_subset_layout_context_t *c) const
   {
     TRACE_SUBSET (this);
-    auto *out = c->subset_context->serializer->embed (*this);
+
-    if (unlikely (!out)) return_trace (false);
+    auto *out = c->subset_context->serializer->start_embed (this);
+    if (unlikely (!c->subset_context->serializer->extend_min (out))) return_trace (false);
+
+    out->version = version;
 
     typedef LookupOffsetList<TLookup, typename Types::HBUINT> TLookupList;
     reinterpret_cast<typename Types::template OffsetTo<TLookupList> &> (out->lookupList)
@@ -4166,9 +4169,23 @@ struct GSUBGPOSVersion1_2
 #ifndef HB_NO_VAR
     if (version.to_int () >= 0x00010001u)
     {
-      bool ret = out->featureVars.serialize_subset (c->subset_context, featureVars, this, c);
+      auto snapshot = c->subset_context->serializer->snapshot ();
+      if (!c->subset_context->serializer->extend_min (&out->featureVars))
+        return_trace (false);
+
+      // TODO(qxliu76): the current implementation doesn't correctly handle feature variations
+      //                that are dropped by instancing when the associated conditions don't trigger.
+      //                Since partial instancing isn't yet supported this isn't an issue yet but will
+      //                need to be fixed for partial instancing.
+
+
+
+      // if all axes are pinned all feature vars are dropped.
+      bool ret = !c->subset_context->plan->all_axes_pinned
+                 && out->featureVars.serialize_subset (c->subset_context, featureVars, this, c);
       if (!ret && version.major == 1)
       {
+        c->subset_context->serializer->revert (snapshot);
 	out->version.major = 1;
 	out->version.minor = 0;
       }

src/hb-subset-plan.cc

@@ -188,8 +188,18 @@ static void _collect_layout_indices (hb_subset_plan_t     *plan,
     f->add_lookup_indexes_to (lookup_indices);
   }
 
+  // If all axes are pinned then all feature variations will be dropped so there's no need
+  // to collect lookups from them.
+  if (!plan->all_axes_pinned)
+  {
+    // TODO(qxliu76): this collection doesn't work correctly for feature variations that are dropped
+    //                but not applied. The collection will collect and retain the lookup indices
+    //                associated with those dropped but not activated rules. Since partial instancing
+    //                isn't yet supported this isn't an issue yet but will need to be fixed for
+    //                partial instancing.
     table.feature_variation_collect_lookups (feature_indices, feature_substitutes_map, lookup_indices);
   }
+}
 
 
 static inline void

test/subset/data/tests/instance_feature_variations.tests

@@ -3,10 +3,10 @@ MPLUS1-Variable.ttf
 
 PROFILES:
 default.txt
-notdef-outline.txt
 
 SUBSETS:
-*
+ポ
 
 INSTANCES:
+wght=100
 wght=400