## Summary of the Pull Request
- Changes the `IRenderEngine` interface to return a vector of values instead of just a single one. Engines that want to report one still can. Engines that want to report multiple smaller ones will be able to do so going forward.
## PR Checklist
* [x] In support of differential rendering #778
* [x] I work here.
* [x] Manually tested it still works.
* [x] Am core contributor.
## Detailed Description of the Pull Request / Additional comments
- Some of my ideas for the `DxEngine` require the ability to specify multiple smaller rectangles instead of one giant one, specifically to mitigate the case where someone refreshes just one cell in two opposite corners of the display (which currently coalesces into refreshing the entire display.)
- This is pulled out into an individual PR to make it easier to review that concept changing.
## Validation Steps Performed
- Ran the Terminal
2b6e96a745