ffmpeg: rename configure_filters() to configure_video_filters()

Mostly useful for the audio-filters branch, to make more apparent the
distinction between configure_audio_filters() and
configure_video_filters().

Signed-off-by: Stefano Sabatini <stefano.sabatini-lala@poste.it>
Signed-off-by: Anton Khirnov <anton@khirnov.net>