Fix WebVR examples with WebXR API

[takahirox]

webvr examples with WebXR API have been broken in dev branch.

Renderer viewport is resized in WebGLRenderer.render() when rendering object with ArrayCamera in VR mode.

https://github.com/mrdoob/three.js/blob/r101/src/renderers/WebGLRenderer.js#L1306-L1321

But viewport size isn't reset when user leaves VR mode.

In r101 .setRenderTarget() resetting viewport inside was called in each .render() call then viewport was automatically reset.

https://github.com/mrdoob/three.js/blob/r101/src/renderers/WebGLRenderer.js#L1106

But with #15571 .setRenderTarget() is no longer called in .render() (unless you give deprecated third argument renderTarget). So we need to reset viewport size when session ends in WebXRManager like WebVRManager does.

Probably adding renderer.setRenderTarget( renderer.getRenderTarget() ); in onSessionEnd() is the easiest solution. Let me know if there is any better ideas.

Update: Plus, I think we need to re-bind framebuffer from session.baseLayer.framebuffer to regular one on session end so calling .setRenderTarget() seems a good solution.

[Mugen87]

Good find! 👍

[mrdoob]

Sounds hacky. But we can improve it later.

[mrdoob]

Thanks!