As long as we can create a GL context, pass one to
gstreamer. This at least gets us GL textures with
the ngl renderer, the previous code was arbitrarily
refusing that.
...for at least WGL, since we are not allowed to use wglMakeCurrent(),
which is eventually called by gdk_gl_context_make_current() to share WGL
contexts across different threads, which will cause a crash.
This means, we cannot enable WGL in the gstreamer media backend on
Windows.
