addPicture method
Adds a Picture to the scene.
The picture is rasterized at the given offset.
Implementation
void addPicture(Offset offset, Picture picture, { bool isComplexHint: false, bool willChangeHint: false }) {
  int hints = 0;
  if (isComplexHint)
    hints |= 1;
  if (willChangeHint)
    hints |= 2;
  _addPicture(offset.dx, offset.dy, picture, hints);
}