Add Block::GetOffset() accessor.

Change-Id: I6172c412a8b2b093a365d634e48fcd7a09363111

Added accessor for Block, to query offset value.

mkvparser.hpp

@@ -80,6 +80,7 @@ public:
     bool IsKey() const;
     void SetKey(bool);
 
+    long long GetOffset() const;
     long GetSize() const;
     long Read(IMkvReader*, unsigned char*) const;