feat(XR): Add base WebXR AR support to OpenGL RenderWindow

[tbirdso]

PR and Code Checklist

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Context

First step in adding AR support as part of our WebXR investigation.

Changes

• Updates OpenGL/vtkRenderWindow to support immersive-ar sessions via the WebXR AR module
• Forks VR cone example for a basic AR example

Results

• AR is functional in WebXR emulator and in mobile device testing

Testing

• All tests complete without errors on the following environment:
  • vtk.js: v21.3.0
  • OS: Windows 10
  • Browser: Chrome 96.0.4664.110
• AR example functions on mobile and in WebXR emulator
  • See WebXR emulator extension repo for install instructions
  • For testing on mobile, run the example with npm test example -- AR and visit the displayed LAN address in your mobile browser. May require adjusting firewall rules on your host machine.

[martinken]

Only thing I would suggest is adding basic (just empty or similar) startXR methods (or some query for the feature) to RenderWindowViewNode which is the superclass for API specific RenderWindows. Otherwise with WebGPU I think it will cause console errors yes?

[tbirdso]

@martinken I haven't specifically tested for WebGPU behavior, are there resources I could use to look into this?

In any case, isn't it reasonable to throw a console error if we try to enter XR on an unsupported backend?

[martinken]

If we only call them when someone is asking to go into VR then sure (which looks like the case here) , though the console message will be that the method doesn't exist as opposed to something nicer. Might be nice to have a query method for if the API supports it so we could hide the toggle. e.g. if on an API (e.g. WebGPU) that doesn't support XR, don;t show the toggle. So even if the browser supports WebXR, if the backend doesn;t support it we don;t show the GUI toggle. It could be something like RenderWindowViewNode.supportsFeature('WebXR') and defaults to false for everything. Then in WebGL we return true for that.

[martinken]

Love it!

[github-actions]

🎉 This PR is included in version 21.5.0 🎉

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