Fix multiview for WebXRManager, some changes were lost in a previous rebase

[vincentfretin]

But I still have a black screen when testing the https://aframe.io/aframe/examples/performance/multiview-extension/?multiview=on example but no errors so there is surely another change needed to make it work again, but no idea what.

@dmarcos there is no "Bump build" commit on the super-r177 branch, you probably did it but not pushed.
I see you fixed a } in super-r177 branch but not in dev, so I included it in this PR.

[vincentfretin]

replaced by #25

[mrxz]

Thanks for testing this, I assumed this would be all that'd be needed to get it working again. After digging into it some more, it seems there were upstream changes in Three.js as well for which the multiview patch needed to be adjusted.

I've created #25 containing the changes you made here along with additional fixes.

[vincentfretin]

Thanks for looking at it. Also current super-three dev is 177, there may have been other changes in three.js dev that may impact the implementation, did you also try to rebase from latest three dev to see if there is anything that would break it again?

[mrxz]

I haven't looked at the changes past 177. Wanted to make sure I could find the cause of the issue first, before mixing in more unknowns/changes. Since the focus in Three.js is shifting towards the new renderer, there's less chances for things to break.

The most important part is verifying that multiview still works after a rebase. That way a regression can be detected earlier, making it easier to find the cause. This time it turned out to be a combination of things lost in rebasing and upstream changes in Three.js.