Some stabilization work

 * Handle the canvas aspect ratio in the output writer not in the render
 * Remove the RotatePoint routine, as it is not used
 * Add proper rendering for animation
 * Use a better visibility routine
 * Use a better depth-sort routine
 * Factorize model-view transormation and back-face culling routines

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>