Draft: Refactor Looking Glass WebXR support

[tbirdso]

Context

Working branch to separate general WebXR behavior from behavior specific to the Looking Glass WebXR polyfill for holographic rendering.

I do not expect to have bandwidth to revisit this work. Others are welcome to pick up this draft PR and carry it to completion. Or, project maintainers are welcome to close the draft PR if it is determined that these changes will not be pursued.

Results

Changes

• Adds vtkWebXRLookingGlassRenderWindowHelper subclass for behavior specific to the Looking Glass WebXR polyfill
• Refactors vtkWebXRRenderWindowHelper to expose getRendererViewport method for subclass override. For instance, the Looking Glass display can override the method to indicate what quilt patch should be drawn for a given view.
• Includes experiments for on-demand Looking Glass rendering, but further efforts would require resolution for Feature Request: Persist Last Rendered Hologram Looking-Glass/looking-glass-webxr#41
• Documentation and TypeScript definitions were updated to match those changes

PR and Code Checklist

• semantic-release commit messages
• Run npm run reformat to have correctly formatted code

Testing

• This change adds or fixes unit tests
• Tested environment:
  • vtk.js:
  • OS:
  • Browser:

fyi @floryst