dix: keep a ref to the rootCursor

CreateCursor returns a cursor with refcount 1 - that refcount is used by
the resource system, any caller needs to call RefCursor to get their own
reference. That happens correctly for normal cursors but for our
rootCursor we keep a variable to the cursor despite not having a ref for
ourselves.

Fix this by reffing/unreffing the rootCursor to ensure our pointer is
valid.

Related to CVE-2025-26594, ZDI-CAN-25544

Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>
(cherry picked from commit b0a09ba602)

Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1830>

dix/main.c

@@ -231,6 +231,8 @@ dix_main(int argc, char *argv[], char *envp[])
             FatalError("could not open default cursor font");
         }
 
+        rootCursor = RefCursor(rootCursor);
+
 #ifdef PANORAMIX
         /*
          * Consolidate window and colourmap information for each screen
@@ -271,6 +273,8 @@ dix_main(int argc, char *argv[], char *envp[])
 
         Dispatch();
 
+        UnrefCursor(rootCursor);
+
         UndisplayDevices();
         DisableAllDevices();