SliverFillRemaining extends beyond viewport size when fillOverscroll is true (#191236)

Overscroll on the trailing edge is reported by the
`constraints.scrollOffset`. Previously, we were using only
`constraints.remainingPaintExtent` to determine the `maxPaintExtent`.
This PR adds `constraints.scrollOffset` to the `maxPaintExtent` to
account for trailing edge overscroll.

In reverse `AxisDirection`s overscrolling at the bottom of the viewport
(which is now the leading edge) stretches the child, increasing its
size. So, this PR uses the child's dynamic size to anchor it to the
viewport edge when overscrolling.

Fixes [SliverFillRemaining(fillOverscroll: true) does not extend beyond
the viewport size](https://github.com/flutter/flutter/issues/141077)