Revert "[subset] Remove ancient hack that I can't reproduce needing anymore"
Revert "[subset] Remove ancient hack that I can't reproduce needing anymore"

This reverts commit 72f72d3c3fff3eea38ae41b86a10a78d8460e09d.

This broken subsetting of cmap, as privately reported by Adam to me.
Investigating some more.  I think I actually know what's going on.

fonttools/fonttools,googlefonts/fonttools