Only interact with the AVSampleBufferDisplayLayer on the main thread

Limelight/Stream/Connection.m

@@ -324,6 +324,7 @@ void ClLogMessage(const char* format, ...)
     LiInitializeVideoCallbacks(&_drCallbacks);
     _drCallbacks.setup = DrDecoderSetup;
     _drCallbacks.submitDecodeUnit = DrSubmitDecodeUnit;
+    _drCallbacks.capabilities = CAPABILITY_DIRECT_SUBMIT;
 
     // RFI doesn't work properly with HEVC on iOS 11 with an iPhone SE (at least)
     // It doesnt work on macOS either, tested with Network Link Conditioner.