This fixes two memory bugs that only manifest with exif=1 and long-running
slideshows.

* when feh loads an image, it writes exif data to file->ed. Previously, this
  data was never free'd, causing a memory leak on subsequent loads of the same
  file.
* As file->ed is never free'd, the accumulated EXIF data consumes a significant
  amount of memory over time. with slideshow-delay = 10 and two days of
  runtime, feh may exceed 1 GB of memory usage. If the slideshow is so large
  that feh does not encounter the same image twice in this time, this is not
  detected as a memory leak, as each EXIF data chunk is referenced from the
  filelist.

See <https://github.com/derf/feh/issues/553> for details.

Closes #553

Issue:

* start feh --multiwindow --reload 5
* close a window with "x"
* wait up to 5 seconds
* segfault

The issue was caused by the closed window's reload timer still being active
even after the winwidget was free'd.

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