coretext: also lazy load fonts based on Family and PostScript names

Previously, the lazy load of fonts was only using display name. Also use the
other names available through the CoreText API (FamilyName and PostScriptName).
coretext: fix conversion from CFStringRef to utf8 buffer

The code incorrectly assumed that the utf8 characters could always be
represented with only one byte. This commit queries CFStringRef instances for
the actual amount of bytes needed.