multiplyTransformMatrices = (a, b) => {
  return [
    a[0] * b[0] + a[2] * b[1],
    a[1] * b[0] + a[3] * b[1],
    a[0] * b[2] + a[2] * b[3],
    a[1] * b[2] + a[3] * b[3],
    a[0] * b[4] + a[2] * b[5] + a[4],
    a[1] * b[4] + a[3] * b[5] + a[5],
  ];
}
Multiply transformation matrices.

@param {ReadonlyArray<number>} a
@param {ReadonlyArray<number>} b
@returns {number[]}