Fix raw highlights clipping

[sevahul]

Description

This PR resolves an issue where high-brightness regions (highlights) in RAW files (such as .NEF) were rendered unnaturally, flattening gradients into white blobs and suffering from "hue twists". It also restores the required HDR headroom (>1.0) so the downstream AgX filmic tonemapper can function correctly without being starved of dynamic range.
In the second commit, a "show original" bug was fixed - the difference is now shown with the selected tone mapper instead of the hard-coded basic one.

Type of Change

• Bug fix

Changes Made

• Restored HDR Headroom: Added a temporary whitelevel = u32::MAX override in develop_internal to bypass rawler's automatic hard-clipping. This preserves >1.0 sensor values created by WB/color-matrix calibration for the downstream WGSL shader.
• Implemented Chromaticity-Preserving Soft-Knee: Replaced the flawed per-channel highlight compression with a smooth, asymptotic soft_knee roll-off mathematical shoulder.
• Fixed Hue Twist: Highlight compression is now applied to the maximum RGB channel (max_c), and the rest of the channels are scaled proportionally (soft_knee(max_c) / max_c). This preserves the precise RGB ratio, keeping saturated lights true to their color instead of twisting toward white.
• Unified Processing: Applied the updated soft_knee scaling logic safely across ThreeColor, Monochrome, and FourColor intermediate formats.

Screenshots/Videos

Before (Clipped Highlights & Hue Twist)	After (Restored HDR Headroom & Gradients)

Testing

• I have tested these changes locally and confirmed that they work as expected without issues

Test Configuration:

• OS: Ubuntu 24.04
• Hardware: Nvidia RTX 2070, Intel

Checklist

• My code follows the project's code style
• I haven't added unnecessary AI-generated code comments
• My changes generate no new warnings or errors (0 Clippy warnings)

Additional Notes

By combining the HDR pass-through (giving the shader proper linear values) with the proportional chromaticity scalar, RapidRAW now accurately matches or exceeds the highlight volume rendering of natural RAW processors like Shotwell.

AI Disclaimer:

Please state the involvement of AI in this PR:

• This PR is entirely AI-generated
• This PR is AI-generated but guided by a human
• This PR was handwritten with AI assistance (spell check, logic suggestions, error resolving)
• This PR contains only blood, sweat, and coffee (AI-free)

The AI indeed generated the above=)

[sevahul]

UPD:

1. The original fix introduced a "magneta highlight effect" - now fixed
2. The current "show original" shows comparison with the "basic" tone mapper. Fix this, it now shows conparison with the selected tone mapper (second commit)

[CyberTimon]

Hi @sevahul

Thanks for your PR!

Are you aware of this option?

In my testing, I could recover much more detail with this slider than with your PR.

Main:

Your PR:

Kind regards,
Timon