code: function getAxis(m, axis, dst) {
    dst = dst || v3.create();
    const off = axis * 4;
    dst[0] = m[off + 0];
    dst[1] = m[off + 1];
    dst[2] = m[off + 2];
    return dst;
  }
docs: Returns an axis of a 4x4 matrix as a vector with 3 entries
  @param {module:twgl/m4.Mat4} m The matrix.
  @param {number} axis The axis 0 = x, 1 = y, 2 = z;
  @return {module:twgl/v3.Vec3} [dst] vector.
  @return {module:twgl/v3.Vec3} The axis component of m.
  @memberOf module:twgl/m4