Add * operator to SingletonPtr

Sometimes you want don't want to directly call a method on your
SingletonPtr-wrapped object, but you want to pass it to something
else.

For example

    SingletonPtr<PlatformMutex> mutex;
    mutex->lock();

is fine, but what about

    SingletonPtr<PlatformMutex> mutex;
    ScopedLock<PlatformMutex> lock(*mutex.get());

Add an overload for operator* to make this more elegant:

    SingletonPtr<PlatformMutex> mutex;
    ScopedLock<PlatformMutex> lock(*mutex);

This addition is consistent with standard C++ classes such as
`unique_ptr` and `shared_ptr`, which likewise have
get, operator-> and operator*.

platform/SingletonPtr.h

@@ -113,6 +113,16 @@ struct SingletonPtr {
         return get();
     }
 
+    /** Get a reference to the underlying singleton
+     *
+     * @returns
+     *   A reference to the singleton
+     */
+    T &operator*()
+    {
+        return *get();
+    }
+
     // This is zero initialized when in global scope
     T *_ptr;
     // Force data to be 4 byte aligned