createQuad/Cylinder/Equirect/CubeLayer init param cannot be optional

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The layers spec shows that the init param to the creation functions for most layer types is optional, defaulting to {}. However each of these init types are inherited from XRLayerInit dictionary, which has three required args:

• space
• viewPixelWidth
• viewPixelHeight

As a result these params should not be marked as optional. It seems like we can probably just remove the optional qualifier from each of them and everything will be fine, since anyone trying to use them without args previously was almost certain to be receiving an error from the browser about not providing all the necessary fields anyway.

Worth noting that createProjectionLayer() SHOULD keep it's init parameter optional, as there's no required fields.

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In making a CL for this I noticed an Issue listed in the spec explaining that this is due to speced/bikeshed#1566, which is still open.

Leaving this issue open for tracking till the root Bikeshed issue is fixed.