[CoreVideo] CVPixelBuffer memory leaks

[chillyjee]

Thank you for the amazing work with SCNRecorder

I'd like to report memory leaks that occur in my case at every recording with version 2.4.0.

Here are screenshots of the memory graph and stack trace:

The profiling was made after sceneView.cancelRecording() was called followed by sceneView de-init.

[v-grigoriev]

Fixed in #25