subpicture: init the position of the PIP region

It's positioned at the center of the display by default as the PIP doesn't set any
alignment or set the absolute flag.

It was the value set by the calloc before 4f0d7e7731.

src/misc/subpicture.c

@@ -164,6 +164,9 @@ subpicture_t *subpicture_NewFromPicture( vlc_object_t *p_obj,
     p_region->fmt.i_sar_num =
     p_region->fmt.i_sar_den = 0;
 
+    // margin to the center or to the top/left if the subpicture is absolute
+    p_region->i_x = 0;
+    p_region->i_y = 0;
 
     vlc_spu_regions_push( &p_subpic->regions, p_region );
     return p_subpic;