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.