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.

Signed-off-by: Adam Jackson <>
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