This works around a regression in Imlib2, which makes (un)loadable file
detection quite slow when handling e.g. large video files. See
<https://phab.enlightenment.org/T8739> and
<https://github.com/derf/feh/issues/505> for details.

Closes #505

Closes #497

On glibc, if output is redirected to a file, output will look like this:

touch test/ok/gif
touch test/ok/jpg
touch test/ok/png
touch test/ok/pnm
test/ok/gif
test/ok/jpg
test/ok/png
test/ok/pnm

On musl, if stdout is redirected to a file, output looks like this:

test/ok/gif
touch test/ok/gif
touch test/ok/jpg
touch test/ok/png
touch test/ok/pnm
test/ok/jpg
test/ok/png
test/ok/pnm

On glibc and musl, if stdout is interactive, it looks like this:

test/ok/gif
touch test/ok/gif
test/ok/jpg
touch test/ok/jpg
test/ok/png
touch test/ok/png
test/ok/pnm
touch test/ok/pnm

Adding two fflush calls makes all behavior look like the last example.
Test cases have been updated accordingly.

Turns out giflib 5.1.2 is the culprit and it isn't Deban-specific after all.

Squashed commit of the following:

commit 58dd2e30
Merge: cf6f0320 82a976e0
Author: Dennis Real <github@tildepipe.org>
Date:   Sat Mar 31 18:43:47 2012 +0200

    Merge branch 'master' of git://github.com/derf/feh

commit cf6f0320
Merge: 10b7b1e3 877e6346
Author: Dennis Real <github@tildepipe.org>
Date:   Sun Mar 25 19:07:30 2012 +0200

    Merge branch 'master' of git://github.com/derf/feh

commit 10b7b1e3
Author: Dennis Real <github@tildepipe.org>
Date:   Sat Mar 24 14:41:15 2012 +0100

    Support for Canon Exif makernote tags

commit 3c284dc8
Author: Dennis Real <github@tildepipe.org>
Date:   Thu Mar 22 19:44:32 2012 +0100

    Support for Active D-Lightning

commit ad5aea8c
Merge: cc6de200 9c43b648
Author: Dennis Real <github@tildepipe.org>
Date:   Thu Mar 22 19:01:38 2012 +0100

    Merge remote branch 'upstream/master'

commit cc6de200
Author: Dennis Real <github@tildepipe.org>
Date:   Thu Mar 22 19:00:30 2012 +0100

    Nikon makernote parts changed

commit e401bd9c
Author: Dennis Real <github@tildepipe.org>
Date:   Wed Mar 21 19:47:15 2012 +0100

    1. Own module for nikon maker notes
    2. Support for AFInfo2

commit 39cbd1ed
Merge: 9e1bbf87 a1001232
Author: Dennis Real <github@tildepipe.org>
Date:   Fri Mar 16 22:22:28 2012 +0100

    Merge remote branch 'upstream/master'

commit 9e1bbf87
Author: Dennis Real <github@tildepipe.org>
Date:   Fri Mar 16 22:20:58 2012 +0100

    - exif handling changed
    - gps info added if available

commit 8e963828
Merge: 77fa5aca 29cd8688
Author: Dennis Real <github@tildepipe.org>
Date:   Fri Mar 16 19:13:47 2012 +0100

    Merge branch 'master' of git://github.com/derf/feh

commit 77fa5aca
Merge: 93a90e60 3b2756bf
Author: Dennis Real <github@tildepipe.org>
Date:   Wed Mar 7 17:18:07 2012 +0100

    Merge branch 'master' of git://github.com/derf/feh

commit 93a90e60
Merge: a81465d0 52d5da9c
Author: Dennis Real <github@tildepipe.org>
Date:   Tue Mar 6 21:42:40 2012 +0100

    Merge branch 'master' of git://github.com/derf/feh

commit a81465d0
Merge: a19e50de 31afac45
Author: Dennis Real <github@tildepipe.org>
Date:   Mon Mar 5 17:23:40 2012 +0100

    Merge branch 'master' of git://github.com/derf/feh

commit a19e50de
Merge: 1c89da12 fe9a16bc
Author: Dennis Real <github@tildepipe.org>
Date:   Sun Mar 4 15:35:45 2012 +0100

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

commit 1c89da12
Author: Dennis Real <github@tildepipe.org>
Date:   Tue Feb 28 22:37:22 2012 +0100

    test case adapted to new testfile

commit bb3c9d35
Author: Dennis Real <github@tildepipe.org>
Date:   Tue Feb 28 22:34:51 2012 +0100

    test file with some exif data added

commit fe9a16bc
Author: Dennis Real <github@tildepipe.org>
Date:   Tue Feb 28 22:37:22 2012 +0100

    test case adapted to new testfile

commit e0bedc04
Merge: 46801fee 69ecc5c1
Author: Dennis Real <github@tildepipe.org>
Date:   Tue Feb 28 22:36:08 2012 +0100

    Merge remote branch 'upstream/master'

commit 46801fee
Author: Dennis Real <github@tildepipe.org>
Date:   Tue Feb 28 22:34:51 2012 +0100

    test file with some exif data added