examples include undefined behavior

[eric-seppanen]

There are several places in the example code that call MaybeUninit::uninit().assume_init(), which is undefined behavior.

Newer toolchains (e.g. rust 1.87.0) print a big warning when this code is compiled:

warning: the type `XSetWindowAttributes` does not permit being left uninitialized
  --> src/main.rs:25:58
   |
25 |         let mut attributes: xlib::XSetWindowAttributes = mem::MaybeUninit::uninit().assume_init();
   |                                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |                                                          |
   |                                                          this code causes undefined behavior when executed
   |                                                          help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
   |
   = note: integers must be initialized
   = note: `#[warn(invalid_value)]` on by default

Values must be initialized before being accessed; MaybeUninit does not excuse the code from that requirement.