Refactor child masking bounds handling

[EVAST9919]

Split from #6261

Original pr has too much stuff changed at once, so a bit more lenient approach feels way better.
Performance shouldn't differ much (if at all).

[EVAST9919]

I believe that this assertion should pass.

The test you've provided failed because as you said children weren't aware of the masking changes in the sub-tree. Invalidating DrawInfo did the trick, and it shouldn't affect performance since there are little to no use cases of frequent masking toggling.

[smoogipoo]

Looks okay now, can't think of anything else that'll break. The DrawNode invalidation is probably not required but I can understand why you've added it (it changes a DrawNode flag). Not a big deal in any case.