Replace std::mem::uninitialized with MaybeUninit (#1027)

* Replace `std::mem::uninitialized` with `MaybeUninit`

* Avoid undefined behavior when using `MaybeUninit`

* Restore unused `PointerState` fields as internally public

* Zero-initialize some struct values in Xlib FFI calls

* Reform usage of `MaybeUninit` in Xlib FFI

* Prefer safe zero-initialization using `Default`, when possible
* Zero-initialize integers and floats using `0` or `0.0`
* Use `MaybeUninit::uninit` for large byte buffers and union types
* Use `MaybeUninit::uninit` when the resulting value is ignored

src/platform_impl/linux/x11/util/mod.rs

@@ -18,7 +18,12 @@ pub use self::{
     randr::*, window_property::*, wm::*,
 };
 
-use std::{mem, ops::BitAnd, os::raw::*, ptr};
+use std::{
+    mem::{self, MaybeUninit},
+    ops::BitAnd,
+    os::raw::*,
+    ptr,
+};
 
 use super::{ffi, XConnection, XError};