Rethink shadow

[LukasKalbertodt]

Currently, shadow is produced by ray marching from the light (or rather: sufficiently far outside of the fractal) to the surface point to check if the path is blocked. This approach has several shortcomings (right now):

• There are many artifacts, probably due to a rather unprecise DE
• The current implementation has quite a few magic constants for fine tuning several values. Reducing the number of parameters would be a good idea.
• It's slow: even with at most 10 iterations, the framerate on my slow notebook GPU drops below 60. Querying the DE a bunch of times in the shader is not the best idea, I guess...

I see three possibilities:

1. improve the current algorithm
2. reverse the direction of the ray: start at the surface point and march towards the light
3. scratch ray marching altogether and use a simple shadow map approach (which might be extremely complicated when "zooming in")