render: delay TIOCSWINSZ while doing an interactive resize

Instead of disabling content centering, delay the TIOCSWINSZ (a.k.a
delay sending the new dimensions to the client) by a small amount
while doing an interactive resize.

Non-interactive resizes are still immediate.

For now, force a resize when the user stops the interactive
resize. This ensures the client application receives the new
dimensions immediately.

It still works without the last, forced, resize, but there typically
be a small delay until the client application receives the final
dimensions.

Closes #301
Closes #283

wayland.h

@@ -390,6 +390,8 @@ struct wl_window {
         bool is_tiled_left:1;
         bool is_tiled_right:1;
     } configure;
+
+    int resize_timeout_fd;
 };
 
 struct config;