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 <ajax@redhat.com>
3 jobs for dmx-fix-glxproxy in 5 minutes and 7 seconds (queued for 29 seconds)
latest
Status Job ID Name Coverage
  Docker Image
passed #846439
debian-testing

00:00:27

 
  Build And Test
passed #846440
autotools-build-and-test

00:04:40

passed #846441
meson-build-and-test

00:02:41