Glx extensions are only interesting to the glxbackend. Given that
querying can be moved there.

In order to simplify the extensions can be stored in the OpenGLBackend
which also provides the convenience check as before.

The egl platforms should also be adjusted to query in that way and
remove it from the kwinglutils.

There is still a usage of the glxextensions inside kwinglutils to
resolve one function. That should also be moved into the platform.