Add 'use-x-over-wayland' feature

Cargo.toml

@@ -41,3 +41,8 @@ wayland-client = { version = "0.8.6", features = ["dlopen"] }
 wayland-kbd = "0.8.0"
 wayland-window = "0.5.0"
 x11-dl = "2.8"
+
+[features]
+# Under linux/bsd use X preferentially over Wayland, if running a Wayland session this means the window will render 
+# using XWayland. This feature will not affect other operating systems.
+use-x-over-wayland = []

examples/window.rs

@@ -3,7 +3,7 @@ extern crate winit;
 fn main() {
     let events_loop = winit::EventsLoop::new();
 
-    let window = winit::WindowBuilder::new()
+    let _window = winit::WindowBuilder::new()
         .with_title("A fantastic window!")
         .build(&events_loop)
         .unwrap();

src/platform/linux/mod.rs

@@ -27,18 +27,27 @@ pub enum UnixBackend {
     X(Arc<XConnection>),
     Wayland(Arc<wayland::WaylandContext>),
     Error(XNotSupported),
-} 
+}
 
 lazy_static!(
     pub static ref UNIX_BACKEND: UnixBackend = {
-        if let Some(ctxt) = wayland::WaylandContext::init() {
-            UnixBackend::Wayland(Arc::new(ctxt))
-        } else {
+        #[inline]
+        fn x_backend() -> UnixBackend {
             match XConnection::new(Some(x_error_callback)) {
                 Ok(x) => UnixBackend::X(Arc::new(x)),
                 Err(e) => UnixBackend::Error(e),
             }
         }
+
+        if cfg!(feature = "use-x-over-wayland") {
+            x_backend()
+        }
+        else if let Some(ctxt) = wayland::WaylandContext::init() {
+            UnixBackend::Wayland(Arc::new(ctxt))
+        }
+        else {
+            x_backend()
+        }
     };
 );