function setAxis(a, v, axis, dst) {
  if (dst !== a) {
    dst = copy(a, dst);
  }

  var off = axis * 4;
  dst[off + 0] = v[0];
  dst[off + 1] = v[1];
  dst[off + 2] = v[2];
  return dst;
}
Sets an axis of a 4x4 matrix as a vector with 3 entries
@param {module:twgl/m4.Mat4} m The matrix.
@param {module:twgl/v3.Vec3} v the axis vector
@param {number} axis The axis  0 = x, 1 = y, 2 = z;
@param {module:twgl/m4.Mat4} [dst] The matrix to set. If not passed a new one is created.
@return {module:twgl/m4.Mat4} The matrix with axis set.
@memberOf module:twgl/m4