demo with 'GENERATED video failed

[CurbShifter]

While checking if this lib I noticed I had to remove this line below to get the demo project to work with 'GENERATED' video output it resets the frame and caused the next WriteFrame call to fail with 'Invalid argument' trown.

https://github.com/Raveler/ffmpeg-cpp/blob/8a6cec3ad2dd629681a2d9c4ee2ba59d594dffe1/source/ffmpeg-cpp/ffmpeg-cpp/Frame%20Sinks/VideoFilterInput.cpp#L22

Removing this call seems to work for the demo. However perhaps this could cause mem leaks elsewhere since it unreferences buffers also. Liking the lib so far though ty!