Thread safety violation in windows::grab::grab

[OpatrilPeter]

Hi, I was randomly browsing your repo and the line

GLOBAL_CALLBACK = Some(Box::new(callback));

struck me. There's no synchronization when modifying this static mut and it's code directly reachable from client code, making it thread unsafe.

I'd recommend to add a mutex or mark that function unsafe. If you plan to keep it safe, I'd also recommend making sure the raw_callback is aware of callback change too. Checking if this idiom isn't used incorrectly elsewhere might help too.