private RectF calculateRectIncludingShadow(RectF rect, DropShadow shadow) {
    if (rectIncludingShadow == null) rectIncludingShadow = new RectF();
    if (tmpRect == null) tmpRect = new RectF();
    rectIncludingShadow.set(rect);
    rectIncludingShadow.offsetTo(rect.left + shadow.getDx(), rect.top + shadow.getDy());
    rectIncludingShadow.inset(-shadow.getRadius(), -shadow.getRadius());
    tmpRect.set(rect);
    rectIncludingShadow.union(tmpRect);
    return rectIncludingShadow;
  }
parentCanvas' pre-existing matrix when start() was called
lottie/src/main/java/com/airbnb/lottie/utils/OffscreenLayer.java