message: fix typo in dirty rect
diff: @@ -234,7 +234,7 @@ export default class Layer extends Eventful {
  const pendingArea = pendingRect.width * pendingRect.height;
  const deltaArea = pendingArea - aArea - bArea;
  if (deltaArea < minDeltaArea) {
  - minDeltaArea = minDeltaArea;
  + minDeltaArea = deltaArea;
  bestRectToMergeIdx = i;
  }
  }
  