The new flags parameter allows to specify if the video ref to add
should overwrite the cache, if the flag is not set vsrc_buffer will
complain and abort; otherwise it will clean the already cached video
ref before to overwrite it, thus avoiding a leak.
The new function is a wrapper around
av_vsrc_buffer_add_video_buffer_ref(), and allows to simplify the act
of pushing AVFrame data to the source buffer.
avfilter_copy_frame_props() avoids code duplication and increases
robustness.
The added files libavfilter/avcodec.[ch] are used for containing
utilities useful for gluing togheter libavfilter and libavcodec.
