Fix WebVR viewport size when presenting

[fernandojsg]

WebVR stereo rendering is currently broken on dev because of same changes I introduced on this PR #16367 (sorry 😢)
Basically I was setting the viewport size correctly on https://github.com/mrdoob/three.js/blob/dev/src/renderers/webvr/WebVRManager.js#L72-L81
*please notice that I was using renderWidth as one eye's width and no the whole render width for both eyes, that's why I multiplied by 2 when calling setDrawingBufferSize https://github.com/mrdoob/three.js/blob/dev/src/renderers/webvr/WebVRManager.js#L78

The problem is that if bounds were provided by the WebVR API, updateViewportFromBounds is being called and it's assuming that renderWidth is the whole render size and not just one eye
https://github.com/mrdoob/three.js/blob/dev/src/renderers/webvr/WebVRManager.js#L185-L193

That's why currently everything is rendering in one eye, and half (1/4 actually) for each eye.

• Tested on Quest, Go and WebVR emulator

[mrdoob]

Thanks!

[mrdoob]

Thanks @toji too for the ping 🙏