[var-common] Make VarInstancer take an offset

src/hb-ot-color-colr-table.hh

@@ -943,10 +943,10 @@ struct ClipBox
       if (u.format == 2 && instancer)
       {
         uint32_t varIdx = u.format2.varIdxBase;
-        xmin += _hb_roundf (instancer (VarIdx::add (varIdx, 0)));
-        ymin += _hb_roundf (instancer (VarIdx::add (varIdx, 1)));
-        xmax += _hb_roundf (instancer (VarIdx::add (varIdx, 2)));
-        ymax += _hb_roundf (instancer (VarIdx::add (varIdx, 3)));
+        xmin += _hb_roundf (instancer (varIdx, 0));
+        ymin += _hb_roundf (instancer (varIdx, 1));
+        xmax += _hb_roundf (instancer (varIdx, 2));
+        ymax += _hb_roundf (instancer (varIdx, 3));
       }
       extents->x_bearing = xmin;
       extents->y_bearing = ymax;

src/hb-ot-var-common.hh

@@ -229,8 +229,8 @@ struct VarStoreInstancer
 
   operator bool () const { return bool (coords); }
 
-  float operator() (uint32_t varIdx) const
-  { return varStore.get_delta (varIdxMap.map (varIdx), coords); }
+  float operator() (uint32_t varIdx, unsigned short offset = 0) const
+  { return varStore.get_delta (varIdxMap.map (VarIdx::add (varIdx, offset)), coords); }
 
   const VariationStore &varStore;
   const DeltaSetIndexMap &varIdxMap;