Correctly decode signed values
Correctly decode signed values

In SV7 these are a mix of signed and unsigned shorts; in SV8 they're all
signed.  Storing them as an int is fine for signed or unsigned shorts as
it's wide enough to contain either of them.

Unfortunately there are no explicit tests for SV7 at the moment; that
would be ideal to add before the next release.

CC @carewolf
taglib/taglib,taglib/taglib,taglib/taglib