Fix calculation of the shadow size in the OpenGL compositor

Summary:
This change addresses the problem of the incorrectly rendered shadows
when opening a window (e.g. Kickoff) a second time. In case of e.g.
Kickoff not all elements are set, thus e.g. left might be 0, but right
has a value. So for calculating the height the maximum of the values
must be used.

Test Plan: Opened Kickoff and systray a few times

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D1792