Define the behavior when destroyed layers are still in the layers array

[cabanier]

@toji mentioned during the WebGPU WebXR layers review that this is currently not defined.
/agenda What should happen if a layer is destroyed but still part of the layers array?

[cabanier]

• destroy should take effect right away
• if the destroyed layer is still in the array, it should be removed by the next frame (ie queue an update to the layers array to remove the layer if it is still available)
• we should block adding a destroyed layer to the layers array or updating the layers array if the destroyed layer is still in there
• getViewSubImage and getSubImage should throw when called on a destroyed layer in both WebGL and WebGPU