added sourceFormatHint for audio input in GPUImageMovieWriter

Make AVAssetWriterInput use the new initializer `+ (AVAssetWriterInput *)assetWriterInputWithMediaType:(NSString *)mediaType outputSettings:(NSDictionary *)outputSettings sourceFormatHint:(CMFormatDescriptionRef)sourceFormatHint NS_AVAILABLE(10_8, 6_0);` which includes the ability to hint at the format of media data that will be appended to the new instance of AVAssetWriterInput.

This gives the GPUImageMovieWriter the ability to passthrough audio for more movie file types, for example mp4.