Fixes incorrect color output when CameraFrame.colorLUT is used with non-trivial LUTs.

LUTs authored in tools like Photoshop and Unreal are encoded in sRGB display space — both their lookup coordinates and stored values are sRGB. The compose pass was indexing the LUT with linear values and treating the sampled sRGB value as linear, producing visibly desaturated and dim output for aggressive LUTs (#8527). With gentle LUTs the mismatch was mostly invisible; with strong tints it was significant.

Changes:

• Compose shader (GLSL and WGSL) now encodes the linear input to sRGB for the LUT lookup coordinate.
• LUT texture is expected to be loaded in an sRGB pixel format. The GPU sampler decodes the sRGB-encoded LUT values to linear, so they can be blended with the linear scene directly.
• Cost: one pow per pixel in the LUT path.
• RenderPassCompose.set colorLUT runs a debug-build validation (wrapped in Debug.call, stripped in release) that checks the texture's srgb, mipmaps, minFilter and magFilter and emits a single Debug.warnOnce listing exactly which settings the LUT needs.

API note:

LUT textures must now be loaded with srgb: true, mipmaps: false, and FILTER_LINEAR for both min and mag filters. Existing code that loaded LUTs with default options will get a one-time debug warning naming exactly what to change, e.g.:

CameraFrame.colorLUT: texture 'colorLut' should be configured with: srgb: true; mipmaps: false; minFilter: FILTER_LINEAR.

Before:

new pc.Asset('lut', 'texture', { url: '...' });

After:

new pc.Asset(
    'lut',
    'texture',
    { url: '...' },
    {
        srgb: true,
        mipmaps: false,
        minfilter: 'linear'
    }
);

In the PlayCanvas Editor, tick sRGB, untick Mipmaps, and set the filter dropdown to Linear on the LUT texture asset.

Examples:

• Updated graphics/hdr example to load its lut-blue asset with the required settings.

Assets:

• Added examples/assets/cube-luts/lut-neutral.png (identity LUT) and lut-cherry.png (aggressive red LUT) for use in a follow-up dedicated LUT example.

Fixed #8527