Allow None for globalSubrs in T2CharString
Handle and warn about out-of-order Coverage tables

Apparently the SIL Gentium fonts have wrong sorting in Coverage
tables.  That's totally broken and the fonts won't work with
HarfBuzz (unless we decide to sort in HarfBuzz sanitize...), but
at least if after sorting it looks sane, nothing is lost.  So,
handle it.