Fix corner screen glows that suddenly pop up

Summary:
Depending on how pointer approaches a screen corner, the maximum
distance between two may be smaller than 2 * edgeDistance. This can
happen for example when approaching the corner along of any two adjacent
screen edges.

As a result, the calculated factor can be anywhere between 0 and 0.5
when pointer enters approachGeometry(). This change adjusts calculation
of the factor, so it always ranges from 0 to 1 no matter how the pointer
approaches corners.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D20707