copyFrom method
Copies the elements from another grid into this grid.
If source is provided, only the elements within the bounds are copied.
A target offset can be provided to copy the elements to a different
position in this grid.
Implementation
void copyFrom(Grid<E> src, {Rect? source, Pos target = Pos.zero}) {
  final dst = this;
  if (source == null) {
    source = src.bounds;
  } else {
    source = bounds.intersect(src.bounds);
  }
  copyRect(
    source,
    src.get,
    dst.set,
    target: target,
  );
}