Add free transformation for lights.

[freezy]

Before this PR, every GameObject had a transformation override, i.e. our custom code handled how the object could be transformed. This was to be sure that the object wasn't moved outside of what the physics engine supports.

However, there are GameObjects like lights that don't concern the physics engine and handling the transformation on our own only complicates things. More concretely, the problem we solved was being able to simply add a LightComponent to existing lights without the override getting confused.

In the future we could apply this to objects that can be freely transformed like primitives as well.

Additionally, code was added to fade emissive materials along with the light, should they be a child of one. It's still hacky because we multiply the alpha value with 0.3f to get somewhat correct results.

Video of it in action:

https://www.youtube.com/watch?v=wLd4VGZUKmU

Lastly, I've updated the documentation and screenshots of the setup process to work with Unity 2021.2.

[codecov]

Codecov Report

Merging #350 (27df9dd) into master (d7525e5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #350   +/-   ##
=======================================
  Coverage   83.91%   83.91%           
=======================================
  Files         125      125           
  Lines        6741     6741           
=======================================
  Hits         5657     5657           
  Misses       1084     1084           

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