revealjs: add visible margin or forced aspect ratio

During the development, pictures are often too large, because the side-by-side preview has a different aspect ratio.

Possible solutions:

- Disable resizing, i.e., always show using a fixed aspect ratio (similar to beamer)
- Optionally, have a visible margin during development that shows that things would be cut off

The issue does not name files, tests, or an entry point. First locate the revealjs preview configuration and compare the available fixed-aspect-ratio and development-margin approaches; done means oversized images no longer make the side-by-side preview misleading.

Written by the indexing model from the issue text.