cursor: reset state if needed on transaction commit

A transaction may move the current target of a cursor action to a
non-visible tag, make it fullscreen, or otherwise change things such
that the current cursor state no longer makes sense.

To handle this, check if we should reset cursor state every time a
transaction is committed.

river/Root.zig

@@ -405,6 +405,11 @@ fn commitTransaction(self: *Self) void {
 
         output.damage.addWhole();
     }
+
+    var seat_it = server.input_manager.seats.first;
+    while (seat_it) |seat_node| : (seat_it = seat_node.next) {
+        seat_node.data.cursor.maybeResetState();
+    }
 }
 
 /// Send the new output configuration to all wlr-output-manager clients