Keep click position in focus on middle-click zoom

Summary:
Clicking on an image in View Mode with the middle mouse button toggles
between Fit and 100% zoom modes. The image zoomed to 100% is
always scrolled to the image center.
This patch keeps the clicked location in focus as it is done while
zooming with Ctrl+Scroll.

FEATURE: 308335

Test Plan:

• Open a big image in View Mode
• Middle-click on different locations over the image
• The 100% zoom view should keep the clicked location in focus

Reviewers: Gwenview, rkflx

Reviewed By: Gwenview, rkflx

Subscribers: rkflx, huoni

Differential Revision: https://phabricator.kde.org/D11336