transcode: add chroma conversion to be the first filter if needed

For example croppadd doesn't like if input and output chromas are not the same.
This is visible for example webcam input and h264 encode (YUV2->I420).

Reported by: Jean-Paul Saman

modules/stream_out/transcode/video.c

@@ -337,6 +337,13 @@ static void transcode_video_filter_init( sout_stream_t *p_stream,
                            p_stream->p_sys );
         filter_chain_Reset( id->p_uf_chain, p_fmt_out,
                             &id->p_encoder->fmt_in );
+        if( p_fmt_out->video.i_chroma != id->p_encoder->fmt_in.video.i_chroma )
+        {
+            filter_chain_AppendFilter( id->p_uf_chain,
+                                   NULL, NULL,
+                                   p_fmt_out,
+                                   &id->p_encoder->fmt_in );
+        }
         filter_chain_AppendFromString( id->p_uf_chain, p_stream->p_sys->psz_vf2 );
         p_fmt_out = filter_chain_GetFmtOut( id->p_uf_chain );
         es_format_Copy( &id->p_encoder->fmt_in, p_fmt_out );