It is a naive way of fixing it :) Needs attentive review

Partial fix of: #310

Addresses part of issue #75

--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.

See #278

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.

This will prevent unnecessary calls to 'feh_calc_needed_zoom'

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/

Closes #351