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

--keep-zoom-vp will no longer block the dynamic window resizing mechanism.

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 #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'