ffmpeg: improve reporting if size/pixel format changes

Use av_log() rather than fprintf(stderr, ...), and show information
related to the previous size/pixel format configuration.

Consistent with the corresponding message issued in case of audio
configuration change.

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