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.

ffmpeg-cpp/source/ffmpeg-cpp/ffmpeg-cpp/Frame Sinks/VideoFilterInput.cpp

Line 22 in 8a6cec3

av_frame_unref(frame);

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!