RotatedRect constructor
Implementation
factory RotatedRect(Point2f center, (double, double) size, double angle) {
  final sz = calloc<cvg.CvSize2f>()
    ..ref.width = size.$1
    ..ref.height = size.$2;
  final ptr = calloc<cvg.RotatedRect>()
    ..ref.center = center.ref
    ..ref.size = sz.ref
    ..ref.angle = angle;
  final rect = RotatedRect._(ptr);
  calloc.free(sz);
  return rect;
}