#16 Support stereoscopic rendering

[Fabrizio Nunnari]

Support for "manual" stereoscopic rendering.
This means that for each CameraObject we need to instantiate 2 Ogre::Cameras.
Provide also the methods to set eyes distance and convergence.
Stereo enabled cameras will have to render on two viewports on th same window.

We should decide wheter to:
- enable the stereoscopic view per camera (a parameter during instantiation, or as attribute) or
- as global TartaViewer parameter (that switches to stereo rendering for any camera). More difficult to implement...