Add a MongolParagraph.dispose() method?

[suragch]

ui.Paragraph has a dispose() method, which TextPainter calls. Should MongolParagraph also add this? I'm not adding it yet because it would require looping through all of the _runs and disposing every Paragraph. Having these runs available potentially allows MongolParagraph to relayout itself when the height changes without having to recalculate every run. That's something that a normal Paragraph doesn't do. So it seems like a performance advantage to not dispose everything. But this is untested. Maybe disposing would be an advantage.