Add Block::GetOffset() accessor.

Change-Id: I6172c412a8b2b093a365d634e48fcd7a09363111

Added accessor for Block, to query offset value.

mkvparser.cpp

@@ -3835,6 +3835,12 @@ void Block::SetKey(bool bKey)
 }
 
 
+long long Block::GetOffset() const
+{
+  return m_frameOff;
+}
+
+
 long Block::GetSize() const
 {
     return m_frameSize;

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;