static L<Rect> gridOfRects3_flattened(int w, int h, int cols, int rows) {
  ret gridOfRects3_flattened(Rect(0, 0, w, h), cols, rows);
}

static L<Rect> gridOfRects3_flattened(Rect r, int cols, int rows) {
  new L<Rect> l;
  for row to rows: {
    int y1 = r.y+iround(r.h*(double) row/rows);
    int y2 = r.y+iround(r.h*(double) (row+1)/rows);
    for col to cols: {
      int x1 = r.x+iround(r.w*(double) col/cols);
      int x2 = r.x+iround(r.w*(double) (col+1)/cols);
      l.add(Rect(x1, y1, x2-x1, y2-y1));
    }
  }
  ret l;
}

Began life as a copy of #1024667