Center image after edit operation changed size

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 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

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

Reviewers: Gwenview, rkflx

Reviewed By: Gwenview, rkflx

Subscribers: rkflx

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