OpenVRDisplay::new should be unsafe

[nox]

OpenVRDisplay::new builds a new OpenVRDisplay out of raw pointers, and then does fancy unsafe things with these raw pointers, which means OpenVRDisplay::new itself should be unsafe.

https://github.com/MortimerGoro/rust-webvr/blob/3bef6ceee33c13ac6b7b6bef4c0eaae13eabf3ea/rust-webvr/src/api/openvr/display.rs#L38-L60