WebXR Fixed Foveation

[Maksims]

The developer can provide a fixed foveation level value (0..1). Underlying rendering system will change the resolution of a framebuffer on the edges of the screens, that way it can improve the rendering performance of fragment shaders, as fewer details will be rendered on the edges.

New APIs:

// pc.XrManager
xr.fixedFoveation // (number|null) a value that can be set and read during an active XR session

Caveat:

Currently, the PlayCanvas engine renders into the intermediate buffer, resolves MSAA and then blits it to the back buffer. With such behaviour, the foveation will not work as the intermediate buffer is not foveated. So currently it only works if no anti-aliasing is enabled for rendering, that way engine renders directly into the back buffer and foveation is beneficial.

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