Fix "translucence" of surfaces

Fix "translucence" of surfaces

The surfaces "glow" instead of reflect light. In other words the lights
and shadows that are seen from one side of a (eq.) plane are seen from
the other (unilluminated) side.
This commit makes it so the light only reflects.
The sphere needs to get self-obstruction implemented. On he inside it
does not cast its own shadow for now.

The test_scene now contains a green light inside the sphere to test the
interior rendering.