this introduces a bug in feh_wm_set_bg, which will be fixed in the next
commit (only applies when run without HOME)

Note that the i == r check was never reached in the original code but may be
in the new variant. Since a shuffled list may well have one or two elements
retain their absolute position once in a while, this should not be a problem.

tests with 2- and 3-element lists show an even distribution amongst all
elements.

Not all functions were examined yet. Also TODO:
* check strdup / malloc calls (some are apparently unchecked in giblib),
  maybe use estrdup / emalloc for those
* Check includes and update dependency list if needed

This means it will be treated like an HTTP image (no write operations,
copying to /tmp etc), but it's still better than no file:// support.

Also, this is why it's not documented yet.

Overloads the --geometry option for wallpaper setting
Currently only supports the two options that result in
black borders, --bg-max and --bg-center.

For both, if a geometry offset is specified (w/h is ignored),
the image will be offset the specified pixels from the screen edge
Left side for + numbers, right side for -.
+/-0 will align at the edge of the screen.

From the XChangeProperty man page:
"If the specified format is 32, the property data must be a long array."

Sort filelist by modification time, newer files first. Useful for
perusing a directory of images by most recently added or changed.