Previously, lifetime management of CTFontRefs was not implemented
correctly. For zero-length ranges, the font may not be retained when
applying it to a CFAttributedString, meaning that the reference stored
in the fontMap sometimes became invalid before
createCFAttributedString() returned.

We now retain font refs when adding them to the font map, and release
them when the map is destroyed, ensuring that the font references remain
valid throughout the lifetime of the map.