xwm: Don't leak pending.input region

It's not really a leak, since the region is always a rectangle, but let's
play by the rules.
diff --git a/src/xwayland/window-manager.c b/src/xwayland/window-manager.c
index ba788ff..9af2dd8 100644
--- a/src/xwayland/window-manager.c
+++ b/src/xwayland/window-manager.c
@@ -827,6 +827,7 @@
 	}
 
 	if (window->surface && !window->fullscreen) {
+		pixman_region32_fini(&window->surface->pending.input);
 		pixman_region32_init_rect(&window->surface->pending.input,
 					  t->margin, t->margin,
 					  width - 2 * t->margin,