ffmpeg: warns the user when the selected pixel format is ignored

Signed-off-by: Anton Khirnov <anton@khirnov.net>

ffmpeg.c

@@ -590,9 +590,16 @@ static void choose_pixel_fmt(AVStream *st, AVCodec *codec)
             if(*p == st->codec->pix_fmt)
                 break;
         }
-        if(*p == -1)
+        if (*p == -1) {
+            if(st->codec->pix_fmt != PIX_FMT_NONE)
+                av_log(NULL, AV_LOG_WARNING,
+                        "Incompatible pixel format '%s' for codec '%s', auto-selecting format '%s'\n",
+                        av_pix_fmt_descriptors[st->codec->pix_fmt].name,
+                        codec->name,
+                        av_pix_fmt_descriptors[codec->pix_fmts[0]].name);
             st->codec->pix_fmt = codec->pix_fmts[0];
         }
+    }
 }
 
 static AVOutputStream *new_output_stream(AVFormatContext *oc, int file_idx)