function fromMat2d(out, a) {
    out[0] = a[0];
    out[1] = a[1];
    out[2] = 0;
    out[3] = a[2];
    out[4] = a[3];
    out[5] = 0;
    out[6] = a[4];
    out[7] = a[5];
    out[8] = 1;
    return out;
  }
Copies the values from a mat2d into a mat3

@param {mat3} out the receiving matrix
@param {ReadonlyMat2d} a the matrix to copy
@returns {mat3} out