Avoid new subgrids from making the Let statements overlap.

Due to the bounding box being wrong, the let statements could overlap.  Fix the
bounding box so as to avoid the issue.


git-svn-id: https://groke.mcmaster.ca/svn/grad/colin/branches/TableTool_refactor@8763 57e6efec-57d4-0310-aeb1-a6c144bb1a8b