[effects/magnifier] Fix 1 px gap between magnified area and frame

Summary:
The x-coordinate of the area's left edge and y-coordinate of the area's
top edge are adjusted by 1px leading to a gap between the frame and
magnified area. Because area has type of QRect, we have to adjust only
coordinates of the right and bottom edge by 1 px.

Because QRectF::right() == QRectF::x() + QRectF::width(), we can use
QRectF, which leads to more cleaner code because we don't need to do 1px
adjustments.

Test Plan:
Before:

After:

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D15275