Correct wxStreamBuffer::Seek() return value.

It returned the offset instead of the new position when seeking forward from
current position in a "flushable" buffer.

Closes #11205.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61958 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/stream.cpp

@@ -635,7 +635,7 @@ wxFileOffset wxStreamBuffer::Seek(wxFileOffset pos, wxSeekMode mode)
                 size_t int_diff = wx_truncate_cast(size_t, diff);
                 wxCHECK_MSG( (wxFileOffset)int_diff == diff, wxInvalidOffset, wxT("huge file not supported") );
                 SetIntPosition(int_diff);
-                return pos;
+                return diff;
             }
 
         case wxFromEnd: