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

multi-window & multi-coordinate system drawing.

There's no new functionality for now: PLEASE TEST THAT EVERYTHING
WORKS AS IT USED TO BEFORE.

trying to fix problem reported by T. Schumacher: use double[16] instead of
double[4][4] for opengl matrices

- make the rotation matrix a matrix of doubles (instead of floats)

- store the modelview and the projection matrices in CTX

- new unproject() routine to transform window coordinates to model
  coordinates

- cleaned up all the static variables in the Opengl_Window class

- small fix to reduce normal clamping when zooming