Add support for EXT_multisampled_render_to_texture extension and use it for WebXR

[cabanier]

The current approach via renderbuffers has the limitation that we can't apply foveation.
Foveation helps a lot for GPU limited scenes so we need to have support for both MSAA and foveation.

EXT_multisampled_render_to_texture is used under the hood in regular WebXR in the Oculus browser and this uses that extension so WebXR Layers (and potential other areas that needs MSAA) can use this.

The WebXR manager was also extended to support this.

This contribution is funded by Oculus.

[mrdoob]

After merging this, do you think we'll be able to remove the current renderbuffers code path any time soon?

[cabanier]

After merging this, do you think we'll be able to remove the current renderbuffers code path any time soon?

I suspect that almost all this code can go away once #22079 is implemented.

[mrdoob]

Excellent!

[mrdoob]

Thanks!

[cabanier]

@mrdoob I have a PR in progress (#22558).
It is working correctly except in the case of multiple render targets and the MSAA extension. With that combination, the content from the other render target only shows in the left eye. Do you know offhand why that would be?

[mrdoob]

Hmm, I can't think of the reason at the moment 🤔

[mrdoob]

Oh, maybe we should implement the stack approach in setRenderTarget() too?

/cc @Mugen87

[cabanier]

So, it's working in the webxr case, non-AA aliased and aliased using renderbuffers.
It just doesn't work using the msaa render-to-texture extension.

[cabanier]

@mrdoob I figured out why it wasn't working. The MSAA extension will discard the depth texture under certain circumstances.
I worked around this in my change.
It's up for review now. Sorry that it got so big :-\

[mrdoob]

@mrdoob I figured out why it wasn't working. The MSAA extension will discard the depth texture under certain circumstances.

Is that a three.js bug? Oculus browser bug? Or WebGL spec bug?

[cabanier]

@mrdoob I figured out why it wasn't working. The MSAA extension will discard the depth texture under certain circumstances.

Is that a three.js bug? Oculus browser bug? Or WebGL spec bug?

It's by design :-(
I'm trying to get it changed.

If texture is a depth or stencil texture, the contents of the multisample
buffer is discarded rather than resolved - equivalent to the application
calling InvalidateFramebuffer for this attachment.

https://www.khronos.org/registry/OpenGL/extensions/EXT/EXT_multisampled_render_to_texture2.txt

[cabanier]

@mrdoob The PR fixes it by using renderbuffers which don't suffer from this issue.
On browser that require the depth texture to be populated, I'm reverting to regular multisampling.

[cabanier]

So, it's working in the webxr case, non-AA aliased and aliased using renderbuffers. It just doesn't work using the msaa render-to-texture extension.

@mrdoob @Mugen87 Should I revert the PR that introduced the extension or patch it so it works?