HDR to SDR v3.1.1

What's new

GPU-only tonemappers

• Added BT.2390 and Spline tonemapper options, available when GPU tonemapping (libplacebo/Vulkan) is enabled.
• Frame preview now renders these tonemappers via GPU-specific extraction, so the preview accurately matches the final output.
• The tonemapper dropdown marks GPU-only options as unselectable when GPU tonemapping is off, and resets the selection automatically if you switch back to CPU.

Fixes

• Prevented invalid filter strings from being built when a GPU-only tonemapper is selected without GPU tonemapping enabled.
• Added validation to block converting a file to itself (same input/output path).

Full Changelog: v3.1.0...v3.1.1