Problems so far:
* leaks zombie processes
* does not work when terminating feh with a signal (since the convert process
  is no longer in feh's process group)

Squashed commit of the following:

commit 55e66e15af898f4c01d7d265013124fefce795e4
Author: Daniel Friesel <derf@finalrewind.org>
Date:   Mon Sep 26 19:59:46 2011 +0200

    Save all filenames in ~/.fehbg

commit afc2774f315fc379aa96de58decf8370772036af
Author: Daniel Friesel <derf@finalrewind.org>
Date:   Mon Sep 26 19:24:38 2011 +0200

    Update test/bg --bg-fill images (off-by-one due to changed implementation)

commit bd4210cff2e895414df12868f6cab7f193c5adf5
Author: Daniel Friesel <derf@finalrewind.org>
Date:   Mon Sep 26 14:47:00 2011 +0200

    Fix feh_wm_set_bg_maxed

commit d9ada9dc2a74aef6ae0924e9c0ff296b94ee25bc
Author: Daniel Friesel <derf@finalrewind.org>
Date:   Mon Sep 26 14:39:11 2011 +0200

    Fix for non-xinerama setups

commit be2cf0f1f8c00b737e7141a372d3cbc9db4c949f
Author: Daniel Friesel <derf@finalrewind.org>
Date:   Mon Sep 26 14:24:44 2011 +0200

    Move wallpaper calculation code into separate functions

commit 4e07c1f9734569fdd778a813c5a85c63b51b3175
Author: Daniel Friesel <derf@finalrewind.org>
Date:   Mon Sep 26 13:01:46 2011 +0200

    Remove unnecessary temporary variable

commit 23d1d733b21d1560e1fbcadddc34cae3d666b21d
Author: Daniel Friesel <derf@finalrewind.org>
Date:   Mon Sep 26 12:46:25 2011 +0200

    Initial conversion, works with --bg-scale

* remove unused variable Im1 in filelist.c
* options are initialised as 0, so only explicitly set those to 0 for which it
  makes sense because of their context
* Make a few warnings about wrong option combinations fatal

Note that read_filelist used to check if its argument was an image (for people
accidentaly typing "feh -df *" instead of "feh -dF *"); this was removed
because it failed on /dev/stdin.

I might re-add it, but I'm not yet sure about the future of feh's automatic
filelist saving.

By default, feh draws white text with a black drop-shadow, which is well
readable in most, but not all cases.  The --draw-tinted option draws a
semi-transparent black box around all texts.  This improves readability, but
does not look as nice, so it won't become default behaviour.

It was suggested to use #7f7f7f7f for the background, but I decided that
for small images surrounded by a black border.

This fixes a really weird bug where continued theme definitions containing
just one option/value pair caused feh to mis-parse parts of the following
line.

Apparently, subsequent realloc calls mixed with strdups in another function
caused parts of the list (argv) content to be overwritten by the content of
the list pointer itself. I wasn't able to find out the exact causes /
conditions.