function rotate$3(out, a, rad) {
    var a0 = a[0],
        a1 = a[1],
        a2 = a[2],
        a3 = a[3],
        a4 = a[4],
        a5 = a[5];
    var s = Math.sin(rad);
    var c = Math.cos(rad);
    out[0] = a0 * c + a2 * s;
    out[1] = a1 * c + a3 * s;
    out[2] = a0 * -s + a2 * c;
    out[3] = a1 * -s + a3 * c;
    out[4] = a4;
    out[5] = a5;
    return out;
  }
Rotates a mat2d by the given angle

@param {mat2d} out the receiving matrix
@param {ReadonlyMat2d} a the matrix to rotate
@param {Number} rad the angle to rotate the matrix by
@returns {mat2d} out