'post' format 4.0 support
use array instead of unpack
More array instead of pack
Merge pull request #103 from olivierberten/post4

'post' format 4.0 support
Minor
Typo
Fix cmap subtable format 4 compiling

Was failing all these years, if a idDelta value happened to be
exactly -0x8000...

Test case: roundtripping MTLmr3m.ttf.
Simplify cmap subtable format 4 idDelta code