Fixed realtime video recording

When realtime video recording,  `newFrameReadyAtTime:atIndex:` and `startRecording` in GPUImageMovieWriter may be called on other thread. 
So `startWriting` in `startRecording` called after `startWriting` in `newFrameReadyAtTime:atIndex:` sometime.
It causes error.
Setting `isRecording` to `YES` after finished `startRecording` fixes issue.