river: destroy backend before Root.deinit()

Handling output destroy now requires the wlr_output_layout to still be
around, as we need it to properly handle cursor state. In order to make
sure that all outputs are destroyed before the wlr_output_layout is,
simply destroy the backend before calling Root.deinit().

river/Server.zig

@@ -139,6 +139,8 @@ pub fn deinit(self: *Self) void {
 
     self.wl_server.destroyClients();
 
+    self.backend.destroy();
+
     self.root.deinit();
 
     self.wl_server.destroy();