Use I420 image buffers instead of YUVA for mosaic bridge. Impact on CPU usage...

Use I420 image buffers instead of YUVA for mosaic bridge. Impact on CPU usage for test case (http://people.videolan.org/~dionoea/mosaic/ basically a 6x5 mosaic displayed locally) is 155% -> 85% if alpha == 255, 166% -> 110% if alpha == 128.

modules/stream_out/mosaic_bridge.c

@@ -438,7 +438,11 @@ static int Send( sout_stream_t *p_stream, sout_stream_id_t *id,
             memset( &fmt_out, 0, sizeof(video_format_t) );
             fmt_in = p_sys->p_decoder->fmt_out.video;
 
+#if 0
             fmt_out.i_chroma = VLC_FOURCC('Y','U','V','A');
+#else
+            fmt_out.i_chroma = VLC_FOURCC('I','4','2','0');
+#endif
 
             if ( !p_sys->i_height )
             {