iOS-GPUImage录制视频第一帧黑屏

使用GPUImage录制视频时第一帧会出现黑屏或者白屏，并且调用addAudioInputsAndOutputs也不好使

此时需要修改GPUImageMovieWriter.m的源码，在其中添加以下代码

static BOOL allowWriteAudio = NO;

- (void)startRecording;
{
  ...
  allowWriteAudio = NO;
}

- (void)processAudioBuffer:(CMSampleBufferRef)audioBuffer;
{
  if (!allowWriteAudio) {
    return;
  }
  ...
}

- (void)newFrameReadyAtTime:(CMTime)frameTime atIndex:(NSInteger)textureIndex;
{
  ...
  if (![assetWriterPixelBufferInput appendPixelBuffer:pixel_buffer withPresentationTime:frameTime])
    NSLog(@"Problem appending pixel buffer at time: %@", CFBridgingRelease(CMTimeCopyDescription(kCFAllocatorDefault, frameTime)));

  allowWriteAudio = YES;   //< add this
  ...
}