this fixes a segfault when combining --scale-down and --thumbnails on a
sufficiently large directory

this actually uses the old "force geometry" workaround to get the
scale down behaviour (which makes the code a lot cleaner) by
first setting the geometry after the window is created (to avoid
creating a 0x0 sized window, which X does not like).

also mark images as removed/processed when clicking on them in thumbnail mode and --action is specifed

slideshow: When removing last image, stay on the last image

The following adds the ability, when using `feh` as a background setter,
to use a specific Xinerama screen to render a provided image. Along with
the provided '--bg-*' option, the '--xinerama-index' argument will be
used to identify the index of the detected Xinerama screen to use.

Signed-off-by: James Knight <james.d.knight@live.com>

The 'fehbg' variable can be allocated whether or not the user wishes to
generated a fehbg file. Ensure the variable is free'ed before we return.

Signed-off-by: James Knight <james.d.knight@live.com>

This way, the current image settings are used when enabling keep_zoom_vp.
It used to reset the current image to default, which probably wasn't very
intuitive.

While at it: Add some notes about .fehbg in general and the fact that it is
an executable shell script now.