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).

This reverts commit b6a1cff6.

This fixes half of the issue - windows on Xinerama screen 0 no longer become
too large, but the bug still appears on the other screens (unless
XINERAMA_SCREEN is set)

Note that feh does not try to convert encodings or anything. It expects
filenames to be in UTF8 and passe them on to NET_WM_NAME and NET_WM_ICON NAME.
The only new thing is that it now also sets _NET_WM_NAME and _NET_WM_ICON_NAME
and explicitly tells X11 that they are in UTF-8. Again, no recode, if you use
UTF-8 filesystems (which you should), everything is fine.

The only thing the patch seems to do is try to calculate the window borders so
it can move the window to the right place. However, XResizeWindow simply does
not move the window no matter how wide the borders are, which is the intended
behaviour. Plus, certain window managers (like fluxbox) report an incorrect
window border width.

I have yet to test this commit on a Xinerama system, though. It _might_ affect
something there.