last table offset add length over file length

[Xxx-Bin]

subheader length add 2 times

[bsweeney]

Noting for reference, this subtable logic performs the following:

• write header
• write content
• calculated subtable length and add the value to the header
• seek back to the header start
• overwrite header with fully populated value
• seek to the position after the table end

We do not need to record the length on the second write because it's overwriting the existing header.