XR on iOS is "swimmy"

[ryantrem]

In XR mode on iOS, objects from the Babylon scene are a bit swimmy. I don't see this issue in other ARKit experiences, so I think this must be a bug in XR.mm. It seems like the camera pose and the camera texture are not quite time synced correctly.

[ryantrem]

This was partially addressed with BabylonJS/BabylonNative#391, but not fully fixed. It is particularly easy to see the issue if you reduce the framerate by adding a std::this_thread::sleep_for(std::chrono::milliseconds(200)); to GetNextFrame in XR.mm.