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 {mat2d} a the matrix to copy
@returns {mat3} out