Fix CVPixelBufferPoolCreatePixelBuffer result comparison crash

[russ-stamant]

Hello.

I fixed the issue I found with the NextLevelSessionExporter when building the framework with Xcode 12.

Using switch with it's default case with let result = CVPixelBufferPoolCreatePixelBuffer causing this exception when running app and using the exporter

** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '*** -[AVAssetWriterInput appendSampleBuffer:] Cannot append sample buffer: Must start a session (using -[AVAssetWriter startSessionAtSourceTime:) first'

I also changed switch result to if result == kCVReturnSuccess since we have nothing left to execute in default case.

[piemonte]

thanks @rshevchuk much appreciated ✌️