Fix crash on repainting an invalid sizes decoration

Summary:
If a window has an invalid size the decoration also has an invalid
size. This results in the texture used by the
SceneOpenGLDecorationRenderer to be invalid and being reset to null.
Of course we shouldn't try to use this texture to render to.

The change comes with a test case to simulate the situation. We cannot
simulate it with Wayland clients as the geometry can never be empty.
Thus we create an X11 client, resize it to an empty size and unmap it.

This is the first integration test case which creates an X11 Client!
It's also a test case which needs the OpenGL compositor. This will most
likely not work on build.kde.org yet - we need to see what to do about
it. Will need adjustments to get it at least skip on build.kde.org.

BUG: 361551
FIXED-IN: 5.6.3

Reviewers: #plasma

Subscribers: plasma-devel

Projects: #plasma

Differential Revision: https://phabricator.kde.org/D1383