Add accessor for underlying display handle on Linux. Although unimplemented on other platforms, this applies to at least android as well.

src/osx/mod.rs

@@ -337,4 +337,8 @@ impl Window {
     pub fn swap_buffers(&self) {
         unsafe { self.context.flushBuffer(); }
     }
+
+    pub fn platform_display(&self) -> *mut libc::c_void {
+        unimplemented!()
+    }
 }