glxproxy: Fix __glXActiveScreens allocation
Apparently this has been broken for about ten years, eesh. We were never allocating any storage for this array, so the first attempt at using GLX with Xdmx would crash the server.
Promote it to an array and use __glXNumActiveScreens to detect whether GLX is actually enabled.
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=34851 Signed-off-by: Adam Jackson firstname.lastname@example.org