Add X11 opt-in function for device events

Previously on X11, by default all global events were broadcasted to
every winit application. This unnecessarily drains battery due to
excessive CPU usage when moving the mouse.

To resolve this, device events are now ignored by default and users must
manually opt into it using
`EventLoopWindowTarget::set_filter_device_events`.

Fixes (#1634) on Linux.

src/event_loop.rs

@@ -286,6 +286,28 @@ impl<T> EventLoopWindowTarget<T> {
     pub fn primary_monitor(&self) -> Option<MonitorHandle> {
         self.p.primary_monitor()
     }
+
+    /// Change [`DeviceEvent`] filter mode.
+    ///
+    /// Since the [`DeviceEvent`] capture can lead to high CPU usage for unfocused windows, winit
+    /// will ignore them by default for unfocused windows on Linux/BSD. This method allows changing
+    /// this filter at runtime to explicitly capture them again.
+    ///
+    /// ## Platform-specific
+    ///
+    /// - **Wayland / Windows / macOS / iOS / Android / Web**: Unsupported.
+    ///
+    /// [`DeviceEvent`]: crate::event::DeviceEvent
+    pub fn set_device_event_filter(&mut self, _filter: DeviceEventFilter) {
+        #[cfg(any(
+            target_os = "linux",
+            target_os = "dragonfly",
+            target_os = "freebsd",
+            target_os = "netbsd",
+            target_os = "openbsd"
+        ))]
+        self.p.set_device_event_filter(_filter);
+    }
 }
 
 /// Used to send custom events to `EventLoop`.
@@ -330,3 +352,20 @@ impl<T> fmt::Display for EventLoopClosed<T> {
 }
 
 impl<T: fmt::Debug> error::Error for EventLoopClosed<T> {}
+
+/// Fiter controlling the propagation of device events.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
+pub enum DeviceEventFilter {
+    /// Always filter out device events.
+    Always,
+    /// Filter out device events while the window is not focused.
+    Unfocused,
+    /// Report all device events regardless of window focus.
+    Never,
+}
+
+impl Default for DeviceEventFilter {
+    fn default() -> Self {
+        Self::Unfocused
+    }
+}