Calculate the intersection only once

The intersection of rects are calculated twice without need.
The function QRectF::intersected() calculate and returns the intersection.
If there are no interesection, the returned QRectF is empty.
If the intersected QRectF is equal to QRectF argument, it contains the QRectF argument.

Reviewed in: https://git.reviewboard.kde.org/r/114036/