fix paths that don't have light contributions

[GoncaloFDS]

The problem:
Paths that do not hit a light are giving a contribution and they shouldn't. e.g If a path it's a wall and terminates if contribution should be zero however with the current code we get that the contribution of that path is the color of the material.

Solution:
if the path exceeds the max path length and it didn't get a contribution from a light, that path contribution is zero.

Path length 1:

Path length 2:

Path length 3:

And some samples gathered from the Ray Tracing: The Rest of Your Life
Path length 1:

Path length 2:

Path length 3:

[GPSnoopy]

Well spotted. I did read the code of the CPU implementations, and you're absolutely right.

Although it only works because the light emitting materials in "RayTracing in one weekend" never scatter rays when hit by them (effectively becoming an implicit stop condition). I've added a comment to that effect.