Takes the lock inside two routines in `TermControl` that were changing
the selection endpoint while a rendering frame was still drawing,
resulting in several variants of graphical glitches from double-struck
selection boxes to duplicated line text.
## References
- Introduced with #5185
## PR Checklist
* [x] Closes#5471
* [x] Already signed life away to company.
* [x] Manual tests passed since it's visual.
* [x] No extra doc besides the comments.
* [x] Am core contributor: Roar.
The renderer base and specific renderer engine do a lot of work to
remember the previous selection and compensate for scrolling regions and
deltas between frames. However, all that work doesn't quite match up
when the endpoints are changed out from under it. Unfortunately,
`TermControl` doesn't have a robust history of locking correctly in step
with the renderer nor does the renderer's `IRenderData` currently
provide any way of 'snapping' state at the beginning of a frame so it
could work without a full lock. So the solution for now is for the
methods that scroll the display in `TermControl` to take the lock that
is shared with the renderer's frame painter so they can't change out of
sync.
## Validation Steps Performed
- Opened terminal with Powershell core.
Did ls a bunch of times.
Clicked to make selection and held mouse button while wheeling around.
- Opened terminal with Powershell core.;
Did ls a bunch of times.
Clicked to make selection and dragged mouse outside the window to make
auto scroll happen.
- Opened terminal with Powershell core.
Did ls a bunch of times.
Clicked to make selection and released. Wheeled around like a crazy
person to make sure I didn't regress that.
98ac196cec