Make Random::next() const, since logically it should be.

The fact that it updates an internal variable is an implementation
detail that shouldn't be exposed outside the class.

src/emucore/Random.hxx

@@ -52,7 +52,7 @@ class Random
 
       @return A random number
     */
-    uInt32 next()
+    uInt32 next() const
     {
       return (myValue = (myValue * 2416 + 374441) % 1771875);
     }
@@ -62,7 +62,11 @@ class Random
     const OSystem& myOSystem;
 
     // Indicates the next random number
-    uInt32 myValue;
+    // We make this mutable, since it's not immediately obvious that
+    // calling next() should change internal state (ie, the *logical*
+    // state of the object shouldn't change just by asking for another
+    // random number)
+    mutable uInt32 myValue;
 
   private:
     // Following constructors and assignment operators not supported

src/emucore/System.hxx

@@ -182,7 +182,7 @@ class System : public Serializable
       @param hmask  The bits which should always be driven high
       @return  The data bus state
     */
-    uInt8 getDataBusState(uInt8 zmask, uInt8 hmask = 0x00)
+    uInt8 getDataBusState(uInt8 zmask, uInt8 hmask = 0x00) const
     {
       // For the pins that are floating, randomly decide which are high or low
       // Otherwise, they're specifically driven high