If mosaic item has an alpha plane keep it even after resizing the picture.

modules/video_filter/mosaic.c

@@ -597,7 +597,11 @@ static subpicture_t *Filter( filter_t *p_filter, mtime_t date )
             fmt_in.i_height = p_es->p_picture->format.i_height;
             fmt_in.i_width = p_es->p_picture->format.i_width;
 
-            fmt_out.i_chroma = VLC_FOURCC('I','4','2','0');
+            if( fmt_in.i_chroma == VLC_FOURCC('Y','U','V','A') ||
+                fmt_in.i_chroma == VLC_FOURCC('R','G','B','A') )
+                fmt_out.i_chroma = VLC_FOURCC('Y','U','V','A');
+            else
+                fmt_out.i_chroma = VLC_FOURCC('I','4','2','0');
             fmt_out.i_width = col_inner_width;
             fmt_out.i_height = row_inner_height;