Warn the user if a pix_fmt != yuv420p was chosen for MPEG-2 video encoding.

Fixes ticket #2870.

ffmpeg.c

@@ -2355,6 +2355,13 @@ static int transcode_init(void)
                            "No pixel format specified, %s for H.264 encoding chosen.\n"
                            "Use -pix_fmt yuv420p for compatibility with outdated media players.\n",
                            av_get_pix_fmt_name(ost->filter->filter->inputs[0]->format));
+                if (!strncmp(ost->enc->name, "mpeg2video", 10) &&
+                    codec->pix_fmt == AV_PIX_FMT_NONE &&
+                    ost->filter->filter->inputs[0]->format != AV_PIX_FMT_YUV420P)
+                    av_log(NULL, AV_LOG_WARNING,
+                           "No pixel format specified, %s for MPEG-2 encoding chosen.\n"
+                           "Use -pix_fmt yuv420p for compatibility with outdated media players.\n",
+                           av_get_pix_fmt_name(ost->filter->filter->inputs[0]->format));
                 codec->pix_fmt = ost->filter->filter->inputs[0]->format;
 
                 if (!icodec ||