compositor: Use a virtual destructor when shutting down

Backend can now rely on their destroy function getting called on shutdown,
so reset tty etc there instead of handling SIGTERM twice.
diff --git a/compositor/compositor.c b/compositor/compositor.c
index 28f16dd..a09f6d9 100644
--- a/compositor/compositor.c
+++ b/compositor/compositor.c
@@ -1452,5 +1452,7 @@
 
 	wl_display_destroy(display);
 
+	ec->destroy(ec);
+
 	return 0;
 }