function rotateHorizontal(camera, angle) {
  if (defined(camera.constrainedAxis)) {
    camera.rotate(camera.constrainedAxis, angle);
  } else {
    camera.rotate(camera.up, angle);
  }
}
Rotates the camera around the center of the camera's reference frame by angle to the left.

@param {number} [angle] The angle, in radians, to rotate by. Defaults to <code>defaultRotateAmount</code>.

@see Camera#rotateRight
@see Camera#rotate