Add [SameObject] to all applicable readonly attributes

[Manishearth]

Fixes #588

There are only two non-primitive readonly attributes that this does not affect:

• XRSession.renderState: This can be updated with updateRenderState()
• XRBoundedReferenceSpace.boundsGeometry: I'm unsure if this array can be updated if settings change

r? @toji

[toji]

These two attributes are the only ones I question in this change, because it implies that each time the render state changes the session attribute will be a new object. That's actually not a bad assumption to make, and it's probably preferable to having XRRenderState be a live object, but I'd like to get that clearly defined one way or the other in the spec prior to adding a dependency on that behavior here.

[Manishearth]

Oh, hm, that's fair. I'll remove that and file an issue

[toji]

Looks great, thank you! I have one change request but otherwise I'm happy to merge this.

You are correct about the boundsGeometry being a live attribute as we have it defined now, by the way.

[Manishearth]

Done.

[Manishearth]

Should I leave a note on XRBoundedReferenceSpace that boundsGeometry may change?

Also I added in two uncommitted SameObject changes that I'd forgotten to push: context and framebuffer on XRWebGLLayer. Does that sound correct?

[toji]

Hm......... context should be safe, but I'm actually not sure about framebuffer. Let me look into that a bit.

[Manishearth]

Yeah, I was a bit iffy about that one too

[toji]

Nope, actually framebuffer should be fine because while the attachments need to swap out on a per-frame basis the framebuffer itself can be a single object throughout! And with that in mind I think I'll merge this now. Thank you!