Sometimes it's useful to apply a paint-time offset to a widget tree. For example, if you have a popup that needs to be positioned relative to another widget, you might want to examine the position of the other widget during the paint phase and offset the popup accordingly.

We can encapsulated this pattern with a widget that calls you back during the paint phase and requests an offset, which it then applies as a paint offset (e.g., similar to a Transform).

/cc @kleak