* Ensure that a frame is allways scheduled when a RenderObject marks itself as needing a semantics update

Fixes issue 2 described in https://github.com/flutter/flutter/issues/10971

Previously, an object could call `markNeedsSemanticsUpdate` and its wish would never be granted because no frame was scheduled that would actually update the semantics. This caused an issue during scrolling on Android where at the end of the scroll the `RenderIgnorePointer` would stop blocking the semantics of the scrolled view, call `markNeedsSemanticsUpdate`, but then no frame was scheduled to actually put the semantics of the scrolled view back into the semantics tree. That made the scrolled view unusable for a11y users.

At first I was a bit wary to call `requestVisualUpdate` within `markNeedsSemanticsUpdate` because technically the visual is fine, we only need the frame it schedules to update the semantics. However, it seems like we are using `requestVisualUpdate` for exactly that purpose in other places already where we just need an update to the semantics (e.g. https://github.com/flutter/flutter/blob/76a50fe0ca9641a835517ca020f9089453989011/packages/flutter/lib/src/rendering/object.dart#L2408).

* add tests