cast_graph cache doesn't

We've just discovered a problem with the cast_graph cache where the
offset in the key used by m_cache.get is not the same as the one used
by m_cache.put. This typically always results in a cache miss so a
large amount of time is spent walking the cast graph.

Fix is a one word change in the function cache::put in inheritance.cpp:

      m_cache.insert(std::make_pair(
          key_type(src, target, dynamic_id, offset)
        , cache_entry(offset, distance)
      ));

where key_type construction should instead use object_offset:

      m_cache.insert(std::make_pair(
          key_type(src, target, dynamic_id, object_offset)
        , cache_entry(offset, distance)
      ));


Which increased our frame rate greatly.


Thanks,
 Chris Byrne

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