gallium: Add destroy callback to all *_winsys interfaces.

For consistency and to simplify these objects' destruction.

src/gallium/drivers/softpipe/sp_screen.c

@@ -139,6 +139,11 @@ softpipe_is_format_supported( struct pipe_screen *screen,
 static void
 softpipe_destroy_screen( struct pipe_screen *screen )
 {
+   struct pipe_winsys *winsys = screen->winsys;
+
+   if(winsys->destroy)
+      winsys->destroy(winsys);
+
    FREE(screen);
 }