Callbacks cause segfaults on windows

[Burgestrand]

Noticed this when I was fixing up Burgestrand/libspotify#5. The examples work fine, as long as no callbacks were called, so the logging in example segfaults before finishing.

Not sure if it’s the casting of managed pointers into the callbacks that is causing it, or just that callbacks do not work well on windows in FFI.

Didn’t have the strength to investigate today, so it’ll have to be at another time.

[Burgestrand]

I did some tries on Windows a month back or so. Had the same issue. Bug is still alive. Didn't do any debugging tho.