This introduces a new feh_should_ignore_image function which is called
at appropriate places in those modes to skip images which are loadable but
undesired.

This will (by default) check the image dimension on-the-fly, but you can still do it beforehand by specifying the preload flag.

This is kinda sloppy coding (feh --filelist and --bg-* will fail when running
on a system with path lengths >= 4096 and PATH_MAX not defined), but that's
sufficiently improbable to be okay. (plus, I ain't getting paid for this, so
if you want to improve it feel free to do so)

Sort filelist by dirname, then by name. This results in file entries
sorting before subdirectory entries.

Useful in conjunction with upcoming prev_dir and next_dir navigation
actions.

manually changed in collage, imlib, filelist, thumbnail, slideshow,
index
feh_display_status also changed

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.

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

commit 9c8226a1
Merge: e4483e5e 132ecc0f
Author: Dennis Real <github@tildepipe.org>
Date:   Sun Feb 5 17:24:17 2012 +0100

    Merge branch 'master' of ssh://github.com/reald/feh

commit e4483e5e
Author: Dennis Real <github@tildepipe.org>
Date:   Sun Feb 5 16:56:29 2012 +0100

    - improved handling of images which have exif tags but not ours
    - fixed uninitialized pointer in thumbnail mode
    - added nikon lens info if available

commit 132ecc0f
Author: Dennis Real <github@tildepipe.org>
Date:   Sun Feb 5 16:56:29 2012 +0100

    - improved handling of images which have exif tags but not ours
    - fixed uninitialzed pointer in thumbnail mode
    - added nikon lens info if available

- fixed uninitialized pointer in thumbnail mode
- added nikon lens info if available

- fixed uninitialzed pointer in thumbnail mode
- added nikon lens info if available

Required e.g. for feh --preload --thumbnails

* 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.