function getBatchColor() {
  const data = this.data;
  const bounds = this.bounds;
  const bw = bounds.w; const bh = bounds.h;
  // calculate batch color
  for (let ii = 0; ii < bw * bh; ++ii) {
    const xx = (ii % bw) | 0;
    const yy = (ii / bw) | 0;
    const px = 4 * (yy * bw + xx);
    if (data[px + 3] <= 0) continue;
    return (data.subarray(px, px + 4));
  };
  return (null);
}
Returns the very first found pixel color
*We expect that the batch is single colored*
@ignore
@return {Uint8Array}