This is a small program that demonstrates various 2D mouse input interfaces for
control a 3D world-in-hand rotation.

So far this implements:
  ROTATION_TYPE_IGL_TRACKBALL  Continuous, "wrap-around" trackball, as seen in
    AntTweakBar.
  ROTATION_TYPE_BELL_TRACKBALL  De facto "Standard" trackball, limited to 180°
    rotations.
  ROTATION_TYPE_TWO_AXIS_VALUATOR   Rotate around drag vector by amount
    proportional to drag length.
  ROTATION_TYPE_TWO_AXIS_VALUATOR_FIXED_UP  Rotate around fixed up-vector by x
    drag length and around y drag vector by y drag length. As seen in Maya, 3D
    Studio Max, etc.

More trackball variantes could be snatched from here:
  http://opentissue.org/svn/OpenTissue/archieve/grid3d/OpenTissue/trackball/
