Summary:
After editing an image which changes its size (crop, resize) or
undoing this operation the image is not centered in the viewport
in some cases.
This patch adds a check if document size changed when calculating
zoom factor and view offset.

FIXED-IN: 18.04.1

Test Plan:
* Open image in View Mode with zoom option {nav Fit} and fit height
* Edit one time to show save bar (e.g. rotate or resize)
* Crop to a narrow upright bar (zoom should not change)
* Undo
* Previous image should display complete and centered

{F5806553}

* Open image in View Mode
* Resize to a size smaller than viewport
* Resize to an even smaller size
* Image should display centered

{F5806555}

Reviewers: #gwenview, rkflx

Reviewed By: #gwenview, rkflx

Subscribers: rkflx

Differential Revision: https://phabricator.kde.org/D12167