OpenXR backend session state is not handled correctly

[chrisfromwork]

Repro:
Open the start menu on a HoloLens 2 while running a BabylonNative application that contains a lot of meshes.

Issue:
OpenXR backend does not correctly tear down rendering when transitioning to an idle OpenXr state. When application perf causes OpenXr to transition to a XR_SESSION_STATE_STOPPING state, BabylonNative ends the OpenXR session. This does not currently ensure that there won't be additional attempts to submit frames, which can cause OpenXr exceptions.