glx: Create the dummy Window with the GLX context

glx: Create the dummy Window with the GLX context

Since we must guarantee that Cogl has a GL context to query, it is too
late to use the "dummy Window" trick from within the get_features()
virtual function implementation.

Instead, we can create a dummy Window from create_context() itself and
leave it around - basically trading a default stage with a dummy X
window.

We need to have the dummy X window around all the time so that the
GLX context can be selected and made current.