seat: set focus before starting transactions

Focus was made double-buffered in 96a91fd. However, much of the code
still behaved as if focus was separate from the transaction system.
This commit completes the work started in 96a91fd and ensures that
focus is applied consistently in a single transaction.

river/InputManager.zig

@@ -149,6 +149,8 @@ fn handleInhibitDeactivate(listener: ?*c.wl_listener, data: ?*c_void) callconv(.
     while (seat_it) |seat_node| : (seat_it = seat_node.next) {
         seat_node.data.focus(null);
     }
+
+    self.server.root.startTransaction();
 }
 
 /// This event is raised by the backend when a new input device becomes available.