Culling: Back face culling and fix polygons.
Polygon:  Now generates Sphere.
Light:  Add direction to spot light
Light:  Add spherical bounding box for positionnal lighting
Light:  Use stencil buffer to only render lit object

The stencil buffer allows to verify if an object is within the light bounding
box instead of just the screen space.  This allows fewer pixel fragments to be
shaded.