Add a way to embed the X11 window into another

Signed-off-by: John Nunley <dev@notgull.net>
Tested-by: Kirill Chibisov <contact@kchibisov.com>

examples/x11_embed.rs

@@ -0,0 +1,69 @@
+//! A demonstration of embedding a winit window in an existing X11 application.
+
+#[cfg(x11_platform)]
+#[path = "util/fill.rs"]
+mod fill;
+
+#[cfg(x11_platform)]
+mod imple {
+    use super::fill;
+    use simple_logger::SimpleLogger;
+    use winit::{
+        event::{Event, WindowEvent},
+        event_loop::EventLoop,
+        platform::x11::WindowBuilderExtX11,
+        window::WindowBuilder,
+    };
+
+    pub(super) fn entry() -> Result<(), Box<dyn std::error::Error>> {
+        // First argument should be a 32-bit X11 window ID.
+        let parent_window_id = std::env::args()
+            .nth(1)
+            .ok_or("Expected a 32-bit X11 window ID as the first argument.")?
+            .parse::<u32>()?;
+
+        SimpleLogger::new().init().unwrap();
+        let event_loop = EventLoop::new();
+
+        let window = WindowBuilder::new()
+            .with_title("An embedded window!")
+            .with_inner_size(winit::dpi::LogicalSize::new(128.0, 128.0))
+            .with_embed_parent_window(parent_window_id)
+            .build(&event_loop)
+            .unwrap();
+
+        event_loop.run(move |event, _, control_flow| {
+            control_flow.set_wait();
+
+            match event {
+                Event::WindowEvent {
+                    event: WindowEvent::CloseRequested,
+                    window_id,
+                } if window_id == window.id() => control_flow.set_exit(),
+                Event::AboutToWait => {
+                    window.request_redraw();
+                }
+                Event::RedrawRequested(_) => {
+                    // Notify the windowing system that we'll be presenting to the window.
+                    window.pre_present_notify();
+                    fill::fill_window(&window);
+                }
+                _ => (),
+            }
+        })?;
+
+        Ok(())
+    }
+}
+
+#[cfg(not(x11_platform))]
+mod imple {
+    pub(super) fn entry() -> Result<(), Box<dyn std::error::Error>> {
+        println!("This example is only supported on X11 platforms.");
+        Ok(())
+    }
+}
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    imple::entry()
+}