[instancer] make GPOS depend on GDEF when partial instancing

GPOS needs to wait for GDEF remapping layout var idxes

src/hb-subset.cc

@@ -461,6 +461,8 @@ _dependencies_satisfied (hb_subset_plan_t *plan, hb_tag_t tag,
   case HB_OT_TAG_vmtx:
   case HB_OT_TAG_maxp:
     return !plan->normalized_coords || !pending_subset_tags.has (HB_OT_TAG_glyf);
+  case HB_OT_TAG_GPOS:
+    return !plan->normalized_coords || plan->all_axes_pinned || !pending_subset_tags.has (HB_OT_TAG_GDEF);
   default:
     return true;
   }