Rust MX Context Object Cannot Be Sent Across Threads
## Summary
As far as I'm aware, GL contexts in MX should be transferrable across threads, so that multithreaded & multi-context GL is achievable in a cross-platform manner.

Currently in the glad2 Rust generator, function pointers are stored as `*const std::ffi::c_void`. The problem is, raw pointers are not `Sync`, which means `FnPtr` (and thus `Gl` in MX) cannot be sent across threads. Although this isn't a critical issue, it's definitely something that should be supported.

## Possible Fix
References are guaranteed to have the same memory layout as their raw pointer alternatives, and *are* `Sync`. So AFAICT, the issue would be no more were all instances of `*const std::ffi::c_void` replaced with `&std::ffi::c_void`.

I'll hopefully get around to submitting a fix PR very soon. Until then, here's a tracking issue. (cc: @Dav1dde)