Rework MonitorId::get_native_identifier (#267)

* Rework MonitorId::get_native_identifier

* Try fix compilation

* Returns the monitor ID on wayland as well

* Try fix compilation

* Fix iOS compilation

src/os/unix.rs

@@ -3,6 +3,7 @@
 use std::sync::Arc;
 use std::ptr;
 use libc;
+use MonitorId;
 use Window;
 use platform::Window2 as LinuxWindow;
 use platform::{UnixBackend, UNIX_BACKEND};
@@ -148,3 +149,16 @@ impl WindowBuilderExt for WindowBuilder {
         self
     }
 }
+
+/// Additional methods on `MonitorId` that are specific to Linux.
+pub trait MonitorIdExt {
+    /// Returns the inner identifier of the monitor.
+    fn native_id(&self) -> u32;
+}
+
+impl MonitorIdExt for MonitorId {
+    #[inline]
+    fn native_id(&self) -> u32 {
+        self.inner.get_native_identifier()
+    }
+}