Aligment improvement.

Small performance increase if the RandomWELL512a_SSE2 is properly SSE
aligned and the single value method is used.

RandomWELL512a_SSE2.h

@@ -45,10 +45,8 @@ public:
 
 private:
     __m128i     state[16];
-    unsigned    index;
-
-    // Helper to allow us to return one number per call.
     unsigned    result[4];
+    unsigned    index;
     unsigned    result_index;
 };