function fromMat4(out, a) {
  out[0] = a[0];
  out[1] = a[1];
  out[2] = a[2];
  out[3] = a[4];
  out[4] = a[5];
  out[5] = a[6];
  out[6] = a[8];
  out[7] = a[9];
  out[8] = a[10];
  return out;
}
Copies the upper-left 3x3 values into the given mat3.

@param {mat3} out the receiving 3x3 matrix
@param {mat4} a   the source 4x4 matrix
@returns {mat3} out