TextInput: handle multiple text inputs correctly

The protocol states that we must send enter and leave to all text input
objects if the client has created multiple.

Only one text input is allowed to be activated by the client per seat
however.

river/InputManager.zig

@@ -197,7 +197,7 @@ fn handleNewInputMethod(
     log.debug("new input method on seat {s}", .{relay.seat.wlr_seat.name});
 
     if (seat.focused.surface()) |surface| {
-        relay.setSurfaceFocus(surface);
+        relay.focus(surface);
     }
 }