GPU: wide-gamut / HDR surface color spaces (and gamma)

[codymullins]

Problem

The surface format is fixed to Bgra8Unorm and there is no color-space / HDR control. Modern displays and media want Display-P3 / Rec.2020 / HDR (e.g. Rgba16Float).

Current state

• src/Skyline.Gpu/WindowSurfaceOptions.cs — format hardcoded to Bgra8Unorm.
• WindowSurface.Capabilities reports supported formats but there's no API to request a color space / HDR format or gamma handling.

What's needed

• Let WindowSurfaceOptions request a color space / HDR format, validated against Capabilities.
• Document gamma/sRGB handling expectations (Skyline stays out of color conversion — that's the app's job).

Acceptance criteria

• A consumer can request P3/HDR where supported and the surface reconfigures; unsupported requests fail cleanly at Configure.