ffmpeg: add support to force specific AVCodecs

previously only codec_ids could be forced, which did not allow
forcing a specific implementation like libopenjpeg vs jpeg2000.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg_opt.c

@@ -785,6 +785,14 @@ static int open_input_file(OptionsContext *o, const char *filename)
         find_codec_or_die(audio_codec_name   , AVMEDIA_TYPE_AUDIO   , 0)->id : AV_CODEC_ID_NONE;
     ic->subtitle_codec_id= subtitle_codec_name ?
         find_codec_or_die(subtitle_codec_name, AVMEDIA_TYPE_SUBTITLE, 0)->id : AV_CODEC_ID_NONE;
+
+    if (video_codec_name)
+        av_format_set_video_codec   (ic, find_codec_or_die(video_codec_name   , AVMEDIA_TYPE_VIDEO   , 0));
+    if (audio_codec_name)
+        av_format_set_audio_codec   (ic, find_codec_or_die(audio_codec_name   , AVMEDIA_TYPE_AUDIO   , 0));
+    if (subtitle_codec_name)
+        av_format_set_subtitle_codec(ic, find_codec_or_die(subtitle_codec_name, AVMEDIA_TYPE_SUBTITLE, 0));
+
     ic->flags |= AVFMT_FLAG_NONBLOCK;
     ic->interrupt_callback = int_cb;