[MovieOutput] Audio buffer is written before video buffer.

[hoangelec]

When I tried recording with MovieOutput, there is occasionally some first black frames in my output video. I guess it's because the audio buffer is written before the video buffer, as mentioned here BradLarson/GPUImage#1255
Looks like it was fix for GPUImage (Obj-C) but not for GPUImage2.
Can you please suggest what I can do to avoid this problem?

[ShivaHuang]

I have patched this issue and created a PR. You can get the modification from #161 or beg @BradLarson to merge.