AK: Expose the memory offset of Vector's outline buffer pointer

Author: https://github.com/awesomekling
Commit: 0bbf230e4f
Pull-request: https://github.com/SerenityOS/serenity/pull/21822
Reviewed-by: https://github.com/DanShaders
Reviewed-by: https://github.com/Hendiadyoin1

AK/Vector.h

@@ -811,6 +811,8 @@ public:
             AK::swap(at(i), at(size() - i - 1));
     }
 
+    static FlatPtr outline_buffer_offset() { return OFFSET_OF(Vector, m_outline_buffer); }
+
 private:
     void reset_capacity()
     {