GPUImageMovieWriter黑屏问题

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
  …
  }**