racket/gui: fix `canvas%' implementation `on-paint'
racket/gui: fix `canvas%' implementation `on-paint'

Broken in the conversion from `class100'.

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