Fix NK_ADD

[tom-huntington]

Some guy in the youtube comments found a bug

your sum operation is not correctly defined. According to the paper

""" All functions appearing in the RandomArt algorithm are scaled so that they map the interval [-1; 1] to the interval [-1; 1]. This condition ensures that all randomly generated expression trees are valid. For example, the scaling for the add function is achieved by defining add(x; y)=(x + y)/2
"""

This property keeps the output colour channels in [-1; 1] and thus stops clipping.

[ChickChicky]

For now it unfortunately doesn't look that nice. Instead of having interesting shapes, it's just a slightly interesting gradient 😢. Hopefully it'll be better with sine / mix / division functions.

[impossibletea]

I believe similar normalization should also be applied to NK_MULT. Multiplication normalization in that case would be sqrtf(lhs->as.number * rhs->as.number).

But this is not that much of an issue, as current implementation of NK_MULT already returns values in $[-1, 1]$.