Add support for foveation

[cabanier]

Closes #22161

Add support for fixed foveation for WebXR experiences.

This contribution is funded by Oculus.

[mrdoob]

Does this need optionalFeatures: ['high-fixed-foveation-level']?

And, just so I understand...
Setting fixedFoveation to 1 means that all the sections of the frame will render at full resolution, right?

[cabanier]

Does this need optionalFeatures: ['high-fixed-foveation-level']?

No. That's an non-standard Oculus-only option.

And, just so I understand...
Setting fixedFoveation to 1 means that all the sections of the frame will render at full resolution, right?

It's the opposite.
0 = no foveation = full resolution
1 = maximum foveation = the edges render at lower resolution

[cabanier]

Most native apps set foveation to the maximum. Should we do the same in three?

[mrdoob]

Thanks!

[mrdoob]

@cabanier

Most native apps set foveation to the maximum. Should we do the same in three?

Sounds good to me 👍