Before this commit, the pptx writer adds a slide break before any table,
“columns” div, or paragraph starting with an image, unless the only
thing before it on the same slide is a heading at the slide level. In
that case, the item and heading are kept on the same slide, and the
heading is used as the slide title (inserted into the layout’s “title”
placeholder).
However, if the slide level is set to 0 (as was recently enabled) this
makes it impossible to have a slide with a title which contains any of
those items in its body.
This commit changes this behaviour: now if the slide level is 0, then
items will be kept with a heading of any level, if the heading’s the
only thing before the item on the same slide.
8e5a79f264