ffi/unsafe win32: fix inefficiency in call-in-orig-thread mode
ffi/unsafe win32: fix inefficiency in call-in-orig-thread mode

The problem made simultaneous rendering of "plot" and "math"
documentation about 10 times slower than it should be.

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