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 {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 none a new one is created
@return {module:twgl/m4.Mat4} dst
@memberOf module:twgl/m4