Update new output handler for wlroots 0.9.0

Following breaking changes in wlroots 0.9.0, wlr_output_commit must be
called after wlr_output_set_mode, wlr_output_set_transform and wlr_output_enable.
Additionally, wlr_output_enable is no longer implicitly called by
wlr_output_set_mode.

Fixes #102

output.c

@@ -401,6 +401,9 @@ handle_new_output(struct wl_listener *listener, void *data)
 	/* Place the cursor in the center of the screen. */
 	wlr_cursor_warp(server->seat->cursor, NULL, wlr_output->width / 2, wlr_output->height / 2);
 	wlr_output_damage_add_whole(output->damage);
+
+	wlr_output_enable(wlr_output, true);
+	wlr_output_commit(wlr_output);
 }
 
 void