Fix a deadlock and a bounding rects issue in UIA (#5385)

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

src/types/UiaTextRangeBase.cpp

@@ -388,7 +388,7 @@ IFACEMETHODIMP UiaTextRangeBase::GetBoundingRectangles(_Outptr_result_maybenull_
 
         // these viewport vars are converted to the buffer coordinate space
         const auto viewport = bufferSize.ConvertToOrigin(_pData->GetViewport());
-        const auto viewportOrigin = viewport.Origin();
+        const til::point viewportOrigin = viewport.Origin();
         const auto viewportEnd = viewport.EndExclusive();
 
         // startAnchor: the earliest COORD we will get a bounding rect for
@@ -426,7 +426,9 @@ IFACEMETHODIMP UiaTextRangeBase::GetBoundingRectangles(_Outptr_result_maybenull_
 
             for (const auto& rect : textRects)
             {
-                _getBoundingRect(rect, coords);
+                til::rectangle r{ rect };
+                r -= viewportOrigin;
+                _getBoundingRect(r, coords);
             }
         }
 
@@ -759,6 +761,8 @@ try
     FAIL_FAST_IF(!(newViewport.Bottom <= bottomRow));
     FAIL_FAST_IF(!(_getViewportHeight(oldViewport) == _getViewportHeight(newViewport)));
 
+    Unlock.reset();
+
     _ChangeViewport(newViewport);
 
     UiaTracing::TextRange::ScrollIntoView(alignToTop, *this);