feat(shaders): added Neupow

[XgarhontX]

Neupow

Neutwo, but instead of "two", the power is a parameter.
Less optimized for more flexibility.
(x * peak) / pow(pow(x, power) + pow(peak, power), rcp(power))
https://www.desmos.com/calculator/693lgrzm2v

• When power = 2: Neutwo.
• When power = 1: Reinhard.
• When power > 2: Similar to late shoulder ExponentialRolloff / GTSport.
• When power < 1: Super compressed & mid gray will noticably shift.

Why?

I'm not a colorist so idk the curve's value regarding that.

I stumbled into by trying to luminance compress HDR color input into SDR LUT.
Being able to flexibly blend between Neutwo and Reinhard greatly help shape HDR upgrade coming out!

Also, as a final HDR tonemapper, it's nice to hook up one white clip parameter and let user tune to their heart's content since the power morphs so it much within just 1 - 4.

No gray in/out versions?

The morph makes everything arbitrary anyways.

Optimization

I let AI optimize with log2/exp2.
I've been using the current version for some newer stuff, and it seems good!

float p_over_x_pow_a = exp2(power * (log2(peak) - log2(x)));
return peak * rcp(exp2(log2(1.0f + p_over_x_pow_a) * rcp(power)));

Invert

I didn't do it yet, and idk if it's worth.

[XgarhontX]

Ok, the log2/exp2 optimization has drawbacks, like really high "probably not going to be used" values causes banding.
Should the accurate pow() functions be used for simplicity & functionality sake?