BUG: Fix rolling functions with variable windows on decreasing index
- [x] Closes https://github.com/pandas-dev/pandas/issues/32385
- [x] tests updated & passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

cc @mroeschke are you familiar with this code? 

I can confirm that indeed the current output (and tested behaviour) is wrong Only familiar from a refactor standpoint. I didn't write the original window bounds calculations. @leftys can you respond to https://github.com/pandas-dev/pandas/pull/32386#pullrequestreview-368778011? Trying to get 1.0.2 out soon. > @leftys can you respond to [#32386 (review)](https://github.com/pandas-dev/pandas/pull/32386#pullrequestreview-368778011)? Trying to get 1.0.2 out soon.

@TomAugspurger done thanks @leftys very nice! @meeseeksdev backport 1.0.x