lottietoqml: Keep Mask nodes visible

In the QML generator, Mask nodes recently got a hardcoded visibility
property value of false. This caused some regressions to our usage of
it to implement mattes, so opt out of that setting.

Pick-to: 6.12
Change-Id: Ie800d2e04c0487caaa8828b41b090bfd2f22c247
Reviewed-by: Hatem ElKharashy <hatem.elkharashy@qt.io>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>