Improve overlays, skybox, color conversion and canting

[TheIronWolfModding]

Improves overlay, skybox, color, and canting handling.

This is mostly LLM port of stuff I did in OC for racing games.

Improves overlay, skybox, color, and canting handling.

• Overlays: OpenVR games can render overlays without calling Submit or PostPresentHandoff (e.g. menu/HUD-only frames), but xrizer's WaitGetPoses only ended a frame after a Submit. Auto-flush pending overlay-only frames when the state ended at Waited so overlay layers actually reach the compositor. Apply matrix diagonal as scale to quad/cylinder size. Cylinder radius = distance-to-user (wraps around head). Fix cylinder aspect_ratio. Treat SetOverlayCurvature(0) as flat quad. Overlay swapchain arraySize=1.
• Skybox: render forward face as curved panel using pTextures[0], always visible.
• Color: manual sRGB decode in overlay shader to avoid double-encoding through SRGB attachment.
• Canting: opt-in XRIZER_PRESERVE_VIEW_CANTING=1 preserves per-eye canted orientations (needed for Pimax etc.).

[galister]

what's the intention behind getting rid of the functional skybox and replacing it with a "curved image in front of the user"? it's called a skybox for a reason.

[TheIronWolfModding]

You are right - years ago OC did not have skybox, so what I did there I simply used front quad only (it is used by racing games (rF2 and Raceroom) to display track loading status). Now, curved simply looks better - allows optionally curved non square texture etc, this is not general, I should not have included it in PR - feel free to abandon.