Bugfix for fitsview, checking wrong width & height in initDisplayImage

Summary:
The check was for the image width & height, but the image can be downsampled in display.
Should check the downsampled widths and heights.

Test Plan: With this bug, the stretch preview was shown 1/4 size. With the bug-fix it should be back to full size.

Reviewers: mutlaqja

Reviewed By: mutlaqja

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D26910