enable conditional compilation for all backends

To reduce binary size one can now enable only specific backends, e.g.
wayland or x11 via cargo features

Additionally adds stubs for libei and xdg-desktop-portal backends

src/backend/consumer/x11.rs

@@ -0,0 +1,49 @@
+use std::{ptr, sync::mpsc::Receiver};
+use x11::{xlib, xtest};
+
+use crate::{
+    client::{Client, ClientHandle},
+    event::Event,
+};
+
+fn open_display() -> Option<*mut xlib::Display> {
+    unsafe {
+        match xlib::XOpenDisplay(ptr::null()) {
+            d if d == ptr::null::<xlib::Display>() as *mut xlib::Display => None,
+            display => Some(display),
+        }
+    }
+}
+
+fn relative_motion(display: *mut xlib::Display, dx: i32, dy: i32) {
+    unsafe {
+        xtest::XTestFakeRelativeMotionEvent(display, dx, dy, 0, 0);
+        xlib::XFlush(display);
+    }
+}
+
+pub fn run(event_rx: Receiver<(Event, ClientHandle)>, _clients: Vec<Client>) {
+    let display = match open_display() {
+        None => panic!("could not open display!"),
+        Some(display) => display,
+    };
+
+    loop {
+        match event_rx.recv().expect("event receiver unavailable").0 {
+            Event::Pointer(pointer_event) => match pointer_event {
+                crate::event::PointerEvent::Motion {
+                    time: _,
+                    relative_x,
+                    relative_y,
+                } => {
+                    relative_motion(display, relative_x as i32, relative_y as i32);
+                }
+                crate::event::PointerEvent::Button { .. } => {}
+                crate::event::PointerEvent::Axis { .. } => {}
+                crate::event::PointerEvent::Frame {} => {}
+            },
+            Event::Keyboard(_) => {}
+            Event::Release() => {}
+        }
+    }
+}