Check that we are indeed running inside an Xorg server before enabling
the workaround.
XWayland or other nested X servers deadlock when that workaround is
applied.
If we want to add an EGL implementation for the X11 backend, we are
going to need to move the GLX bits into their own class. The first step
is to declare GdkX11GLContext as an abstract type, and then subclass it
into a GdkX11GLContextGLX type, which includes the whole GLX
implementation.
