hiding image with overlays; bounds issue

hiding image with overlays; bounds issue

[Psionic K]

Dslide needs to dynamically show and hide content.  I have been
relying on overlays just for ease of accounting.  Text properties are
a valid fallback if I decide overlays are too fiddly.

I've run into a few cases where if I don't extend the overlay to
underlap the beginning of an image, the overlay does not result in the
image being hidden.  Another case is a bit more egregious, but I
couldn't put the example into dslide's demo.org file.

I can probably diagnose this later with fresh eyes, but at this time,
I am in need of freeing some biological memory and wanted to ask in
case it's obvious what likely went wrong.

The offending code hunks are the calls to `1-` here in
https://github.com/positron-solutions/dslide/commit/ef9b7fe09551e0498a174a3ffa5b48b465cb1a07

Btw, this will be in the next dslide release that I am about to cut.
Some cool features got added.  Available in master and I appreciate
running the demo.org to smoke out issues before I tag the release.