X11: Sync key press/release with window focus (#1296)

* X11: Sync key press/release with window focus

* When a window loses focus, key release events are issued for all pressed keys
* When a window gains focus, key press events are issued for all pressed keys
* Adds `is_synthetic` field to `WindowEvent` variant `KeyboardInput`
  to indicate that these events are synthetic.
* Adds `is_synthetic: false` to `WindowEvent::KeyboardInput` events issued
  on all other platforms

* Clarify code with comments

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

@@ -0,0 +1,92 @@
+use std::{iter::Enumerate, ptr, slice::Iter};
+
+use super::*;
+
+pub struct Keymap {
+    keys: [u8; 32],
+}
+
+pub struct KeymapIter<'a> {
+    iter: Enumerate<Iter<'a, u8>>,
+    index: usize,
+    item: Option<u8>,
+}
+
+impl Keymap {
+    pub fn iter(&self) -> KeymapIter<'_> {
+        KeymapIter {
+            iter: self.keys.iter().enumerate(),
+            index: 0,
+            item: None,
+        }
+    }
+}
+
+impl<'a> IntoIterator for &'a Keymap {
+    type Item = ffi::KeyCode;
+    type IntoIter = KeymapIter<'a>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.iter()
+    }
+}
+
+impl Iterator for KeymapIter<'_> {
+    type Item = ffi::KeyCode;
+
+    fn next(&mut self) -> Option<ffi::KeyCode> {
+        if self.item.is_none() {
+            while let Some((index, &item)) = self.iter.next() {
+                if item != 0 {
+                    self.index = index;
+                    self.item = Some(item);
+                    break;
+                }
+            }
+        }
+
+        self.item.take().map(|item| {
+            debug_assert!(item != 0);
+
+            let bit = first_bit(item);
+
+            if item != bit {
+                // Remove the first bit; save the rest for further iterations
+                self.item = Some(item ^ bit);
+            }
+
+            let shift = bit.trailing_zeros() + (self.index * 8) as u32;
+            shift as ffi::KeyCode
+        })
+    }
+}
+
+impl XConnection {
+    pub fn keycode_to_keysym(&self, keycode: ffi::KeyCode) -> ffi::KeySym {
+        unsafe { (self.xlib.XKeycodeToKeysym)(self.display, keycode, 0) }
+    }
+
+    pub fn lookup_keysym(&self, xkev: &mut ffi::XKeyEvent) -> ffi::KeySym {
+        let mut keysym = 0;
+
+        unsafe {
+            (self.xlib.XLookupString)(xkev, ptr::null_mut(), 0, &mut keysym, ptr::null_mut());
+        }
+
+        keysym
+    }
+
+    pub fn query_keymap(&self) -> Keymap {
+        let mut keys = [0; 32];
+
+        unsafe {
+            (self.xlib.XQueryKeymap)(self.display, keys.as_mut_ptr() as *mut c_char);
+        }
+
+        Keymap { keys }
+    }
+}
+
+fn first_bit(b: u8) -> u8 {
+    1 << b.trailing_zeros()
+}

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

@@ -9,6 +9,7 @@ mod geometry;
 mod hint;
 mod icon;
 mod input;
+pub mod keys;
 mod memory;
 pub mod modifiers;
 mod randr;