Improve AgX tonemapping curve and update tonemapping docs.

[allenwp]

• Fixes AgX: Oscillations in sigmoid approximation #102416
• Does NOT fix issue AgX has low dynamic range output with Mobile renderer #101558, but adds a note to the documentation about this limitation.

Not cherry-pickable to 4.3, as AgX is only in 4.4.

Draft State

I have left this PR in a draft state solely to reduce the review workload; I hope that I will be able to greatly improve performance with a new approximation of the AgX tonemapping curve in linear encoding, but it will take me at least a few more weeks of full-time research and trial-and-error to do this.

If the production team would prefer to review this PR as-is and merge it now, you're very welcome to! The new high performance approximation that I have in mind could be merged at a later time.

Tonemapping Curve

This PR changes the tonemapping curve to no longer be an approximate, but instead be an exact simplification of the original tonemapping curve of AgX. This addresses all of the issues described in #102416, while maintaining similar performance.

This PR	Current Approximation

Performance

Using the visual profiler, Calinou's tonemapping test scene, ~4K window, and an NVIDIA 980 Ti on Windows 11, I recorded the following performance stats:

AgX (with white parameter): 1.35 ms
Tony McMapface: 1.29 ms
AgX (this PR): 1.22 ms
AgX (Godot 4.4 beta 2): 1.19 ms
ACES: 1.07 ms
Filmic: 1.0 ms
Rienhard: 0.93 ms
Linear: 0.82 ms

Docs

The docs changes make the tonemapper descriptions consistent with each other, making them easier to quickly compare. They follow this format:

1. Description of tonemapping curve.
2. Key properties and their impact on the resulting image.
3. Performance characteristic relative to other tonemappers.
4. Additional notes, if required.

I have also updated the white documentation along with this refresh.