replace OnceLock with AtomicPtr in NATIVE_DISPLAY

[joseluis]

I wanted to be able to start and stop a miniquad context and after trying several solutions to make NATIVE_DISPLAY droppable (e.g. using RwLock), I believe this one with AtomicPtr plus an static AtomicBool has a very good balance between performance and security.

A simple test:

use miniquad::*;

struct Stage {
    ctx: GlContext,
}
impl EventHandler for Stage {
    fn update(&mut self) {}
    fn draw(&mut self) {}
}
fn main() {
    println!("first context\nclose the window to create a second context.");
    miniquad::start( conf::Conf::default(), || { Box::new(Stage { ctx: GlContext::new(), }) });
    println!("second context");
    miniquad::start( conf::Conf::default(), || { Box::new(Stage { ctx: GlContext::new(), }) });
}

I also got the idea of the native_display function wrappers from #505 to improve ergonomics.

EDIT: This commit would effectively lower MSRV to 1.64. Related: #524.