API: reimplement FixedWindowIndexer.get_window_bounds to fix groupby bug
- [x] closes #36040
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Creating PR to solve [Issue 36040](https://github.com/pandas-dev/pandas/issues/36040).

The old `FixedWindowIndexer.get_window_bounds` function provided unintuitive bounds that went beyond the length of the original array. Additionally, to do a centered rolling operation, it required NaN values to be appended to the end of original array to enable some roundabout way of achieving the centering.

I replaced it with one that seems much simpler and actually creates "fixed" size windows (at least prior to clipping the ends), which the previous function did not.

That being said, I know this PR fails some tests, I'm would appreciate some advice on how best to proceed!