GPUImageMovieWriter: synchronise finishRecording with the movieWritingQueue

As the AVAssetWriter gets started on the movieWritingQueue.

Also ensure that the handler is called async, otherwise it is called on the movieWritingQueue, which is dispatch_sync from the video processing queue, so if the user-code in the handler calls anything that dispatch_syncs to the video processing queue: deadlock.
Merge pull request #1176 from karlvr/movie-writer-finish-sync2

GPUImageMovieWriter: synchronise finishRecording with the movieWritingQueue [resubmit]