Remove Xrender backend
See also https://mail.kde.org/pipermail/kwin/2021-June/005232.html
The Xrender backend was added at the time when OpenGL drivers were not particularly stable. Nowadays though, it's a totally different situation.
The OpenGL render backend has been the default one for many years. It's quite stable, and it allows implementing many advanced features that other render backends don't.
Many features are not tested with it during the development cycle; the only time when it is noticed is when changes in other parts of kwin break the build in the xrender backend. Effectively, the xrender backend is unmaintained nowadays.
Given that the xrender backend is effectively unmaintained and our focus being shifted towards wayland, this change drops the xrender backend in favor of the opengl backend.
Besides being de-facto unmaintained, another issue is that QtQuick does not support and most likely will never support the Xrender API. This poses a problem as we want thumbnail items to be natively integrated in the qtquick scene graph.