racket/draw: fix problem with caching of unscaled, single characters
racket/draw: fix problem with caching of unscaled, single characters

Ccahing retains a "run" layout that is destroyed along with its
iterator.

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket