GPUImageMovieWriter: finishRecordingWithCompletionHandler must happen synchronously on video processing queue too

finishRecording runs synchronously, but if you call the method with a completion handler you don't. This results in a race condition to finish recording.
Merge pull request #1088 from karlvr/movie-writer-finishRecordingWithCompletionHandler

GPUImageMovieWriter: finishRecordingWithCompletionHandler must happen synchronously on video processing queue too