Fix zoom gesture focal point

Currently, the pinch zoom gesture zooms into a wrong focus point (it
uses the screen position instead of the widget local position). This
fixes it to actually zoom into the point your touch position is located
at.

Differential Revision: https://phabricator.kde.org/D8167

(cherry picked from commit c34fbd1976b2ca852b170a31ff0e764dd6ccc5b4)