videogenerator: use AVPixelFormat

src/media/media_decoder.cpp

src/media/media_decoder.h

src/media/video/video_base.h

src/media/video/video_input.cpp

src/media/video/video_input.h

src/media/video/video_mixer.cpp

src/media/video/video_mixer.h

src/media/video/video_receive_thread.cpp

src/media/video/video_receive_thread.h