dix: cache ResourceClientBits() value

The `LimitClient` is set once and for all at startup, whereas the
function `ResourceClientBits()` which returns the client field offset
within the XID based on the value of `LimitClient` can be called
repeatedly.

Small optimization, cache the result of `ilog2()`, that saves running
the same loop over and over each time `ResourceClientBits()` is called.
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>

dix/resource.c

@@ -620,7 +620,12 @@ ilog2(int val)
 unsigned int
 ResourceClientBits(void)
 {
-    return (ilog2(LimitClients));
+    static unsigned int cached = 0;
+
+    if (cached == 0)
+      cached = ilog2(LimitClients);
+
+    return cached;
 }
 
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