This is a second attempt to merge #107269. Currently I've fixed two of the issues:
1. Fixed horizontal scrollview by using a switch statement to consider vertical/horizontal case.
2. Fixed issue of `paintExtent` not being the right extent for painting. Rather using a `scrollExtent` for the main axis length of the decoration box and painting it offsetted by the `scrollOffset`.
3. If the sliver child has inifinite scrollExtent, then we only draw the decoration down to the bottom of the `cacheExtent`. The developer is expected to ensure that the border does not creep up above the cache area.

This PR includes a test that checks that the correct rectangle is drawn at a certain scrollOffset for both the horizontal and vertical case which should be sufficient for checking that `SliverDecoration` works properly now.

Fixes https://github.com/flutter/flutter/issues/107498.