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>

ffmpeg.c

@@ -1192,8 +1192,11 @@ static void do_video_out(AVFormatContext *s,
     if (   ost->resample_height != ist->st->codec->height
         || ost->resample_width  != ist->st->codec->width
         || (ost->resample_pix_fmt!= ist->st->codec->pix_fmt) ) {
-
-        fprintf(stderr,"Input Stream #%d.%d frame size changed to %dx%d, %s\n", ist->file_index, ist->index, ist->st->codec->width,     ist->st->codec->height,avcodec_get_pix_fmt_name(ist->st->codec->pix_fmt));
+        av_log(NULL, AV_LOG_INFO,
+               "Input stream #%d.%d frame changed from size:%dx%d fmt:%s to size:%dx%d fmt:%s\n",
+               ist->file_index, ist->index,
+               ost->resample_width  , ost->resample_height  , avcodec_get_pix_fmt_name(ost->resample_pix_fmt),
+               ist->st->codec->width, ist->st->codec->height, avcodec_get_pix_fmt_name(ist->st->codec->pix_fmt));
         if(!ost->video_resample)
             ffmpeg_exit(1);
     }