Passing an empty string to the --geometry option will enable fixed geometry mode without having to specify anything else

This simplifies the logic behind the automatic zoom ratio calculation, which is used by both --auto-zoom and --scale-down.

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

Closes #349

See #278

Closes #386

Merge branch 'fix-toggle-fixed-geometry' of https://github.com/ulteq/feh into ulteq-fix-toggle-fixed-geometry

Closes #272

Closes #367

Closes #218

Merge branch 'fix-no-files-found-on-reload' of https://github.com/Ferada/feh into Ferada-fix-no-files-found-on-reload

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