In the reveal-js output, it’s possible to use reveal’s
`data-background-image` class on a slide’s title to specify a background
image for the slide.
With this commit, it’s possible to use `background-image` in the same
way for pptx output. Only the “stretch” mode is supported, and the
background image is centred around the slide in the image’s larger axis,
matching the observed default behaviour of PowerPoint.
- Support `background-image` per slide.
- Add tests.
- Update manual.
