Global Illumination

[Negrini085]

This pull request aims to enhance our path tracer by introducing a direct light sampling. The current approach is to treat lights as ObjectHandlers with a specific flag. In this way we correctly calculate if an object is in direct line with a light when traversing the Scene.tree.

The work that needs to be done:

• Surface Sampling: how to importance sampling for surface lights.
• Choose and implement heuristic for weighting multiple lights: emitted power, surface/distance...
• Update example of Cornell box with same renderer parameters.

Some resources:

• https://pbr-book.org/3ed-2018/Light_Transport_I_Surface_Reflection/Direct_Lighting
• https://en.wikipedia.org/wiki/Global_illumination