iOS NativeCamera session start/stop synchronization (#1137)

* Fix race condition between engine shutdown and readTextureAsync.

* Create a serial queue for the camera session start/stop, pre-emptively release texture handle before resolve in readTextureAsync .

* Move texture cache deallocation outside of dispatch_async.

* Whitespace

* Remove rare hang due to unncessary thread marshalling.  Hold onto texture across sessions (hacky fix)

* Change to arcana background dispatcher, re-use implData object, remove unncessary main queue dispatch.

* Consolidate first time initialization code.

* Revert NativeEngine workaround.

* Reuse capture session and camera texture delegate

* Revert keeping camera texture delegate alive, due to deadlocks when reusing dispatch_queue across sessions.

* Move orientation de-register to dealloc, clean up a few comments, clear refreshBgfxHandle

* Cleanup

* Push temporary readTexture fix

* Fix Mac OS build errors

* use shared_from_this().

* Perform one final texture cache check before assigning in delegate.

Author: Alex-MSFT

Plugins/NativeCamera/Source/Apple/NativeCameraImpl.h

@@ -34,5 +34,7 @@ namespace Babylon::Plugins
         bool m_overrideCameraTexture{};
 
         CameraDimensions m_cameraDimensions{};
+
+        arcana::background_dispatcher<32> m_cameraSessionDispatcher{};
     };
 }