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.

compositor/compositor.c

@@ -1452,5 +1452,7 @@ int main(int argc, char *argv[])
 
 	wl_display_destroy(display);
 
+	ec->destroy(ec);
+
 	return 0;
 }