message: Fix background color with gl layer
diff: @@ -267,7 +267,8 @@ Painter.prototype = {
  var z = zlevelList[i];
  var layer = this._layers[z];
  if (!layer.__builtin__ && layer.refresh) {
  - layer.refresh();
  + var clearColor = i === 0 ? this._backgroundColor : null;
  + layer.refresh(clearColor);
  }
  }
  @@ -422,7 +423,8 @@ Painter.prototype = {
  var useTimer = !paintAll && layer.incremental && Date.now;
  var startTime = useTimer && Date.now();
  - var clearColor = k === 0 ? this._backgroundColor : null;
  + var clearColor = layer.zlevel === this._zlevelList[0]
  + ? this._backgroundColor : null;
  // All elements in this layer are cleared.
  if (layer.__startIndex === layer.__endIndex) {
  layer.clear(false, clearColor);
  