Set --slideshow-delay to a negative value to achieve that. In that case, the
actual slideshow-delay will be <value> * (-1), so -D-5 sets slideshow-delay 5
seconds but starts paused.

Closes: GH-4 <http://github.com/derf/feh/issues#issue/4>

This makes the tests also work on systems which don't run X.

See ChangeLog / man/feh.1 "--thumb-redraw" for more

Depending on --thumb-width/--thumb-height, either .thumbnails/normal or
.thumbnails/large is used. For higher dimensions, thumbnail caching is still
silently disabled, this should also be what the standard indicates.

(This was actually meant to be included in
f4420709)