Merge branch 'custom-background-color' of https://github.com/ulteq/feh into ulteq-custom-background-color

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.

Merge branch 'dimension-pruning-without-preload' of https://github.com/ulteq/feh into ulteq-dimension-pruning-without-preload

Closes #381

FWIW: This behaviour was already present before
394517d1

* 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.

Only calculate required zoom ratio when necessary

This will prevent unnecessary calls to 'feh_calc_needed_zoom'