Closes #400

Closes #279

Addresses part of issue #75

This restores pre-2.21 ~/.fehbg behaviour. Replicating the entire commandline
in ~/.fehbg turned out to do more harm than good.

Closes #398

Fixes non-random behaviour when randomizing file lists several times per
second.

Closes #349

See #278

Closes #218

Closes: #382, #384

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.

Closes #381

* Prevents nasty loading loops

* Prevents zombie subprocesses

* Fixes the conversion timeout detection routine

Halves the start-up time of the slideshow if the title contains data from 'file->info'

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

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.

Fixes issue 325.

This will prevent unnecessary calls to 'feh_calc_needed_zoom'

This option allows you to change the default imlib2 image cache size of 4 MiB.

This prevents removing the image data from the cache, when moving back and forth between images. As suggested by the Imlib documentation: http://alien.cern.ch/cache/imlib2-1.0.6/doc/

might resolve #333 and #348

Closes #351