Modification for introduction of stereo mode in

utils/api_demos/mainGlut  :

- introduction of a class Camera to use function glulookat() instead of
rotation matrix in openGL viewport.

- A special implementation of the Trackball for stereo mode to be
consistent with other code (Abaqus, Catia, ...)

- adaptation of Drawcontext in Camera mode (works for mainGlut but to
  be validated in gmsh