Consider replacing "list of pending render states" with "pending render state"

[ddorwin]

Having a list just adds a little more complexity and potential confusion for the user, so it'd be nice to collapse it if possible.

It seems that updateRenderState() could maintain a single nullable "pending render state" that gets updated (as in step 5 of https://immersive-web.github.io/webxr/#apply-pending-render-states) and when not null gets applied in https://immersive-web.github.io/webxr/#apply-pending-render-states.

[toji]

Editors talked about this and remembered that the current design was due to a previous design that had us resolving a promise every time a change was applied. That's not longer a concern, however, so this simplification is fine.

[Manishearth]

#701