osx: Fix calling [window center] before setting gl_context

[birhburh]

For

cargo +nightly run --example=quad                  

I recieved

    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.02s
     Running `target/debug/examples/quad`

thread 'main' panicked at src/native/macos.rs:304:13:
null pointer dereference occurred
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread caused non-unwinding panic. aborting.
zsh: abort      cargo +nightly run --example=quad

Also decided to move some functions below gl_context initialization just to be sure that everything is inited before creation of view and window
This probably could fix some other osx bugs with mouse input

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Why I needed nightly compiler for miniquad is because I tried to use glam with simd feature, but it is probably not important

[not-fl3]

Thanks for PR!