The scroll locking rework that landed with the DxRenderer's partial
invalidation change introduced a deadlock. UIA locks the buffer for
reading before asking it to scroll (which now requires a write lock.)
Scrolling is probably _okay_ to have a little bit of torn state that
might arise from us unlocking the read lock early before triggering the
write lock down the line.
While investigating this, I also noticed that our bounding rects stopped
being viewport-relative (and were instead buffer-relative.) That
regressed with the switch to `GetTextRects` in #4991
## PR Checklist
* [ ] Closes (issues noticed in investigating the DPI changes)
* [x] CLA signed.
* [ ] Tests added/passed
* [ ] Requires documentation to be updated
* [x] Core contributor badge
## Validation Steps Performed
Manual pass with inspect.exe
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