expose the platform-specific window handle (currently Win only)

src/win32/mod.rs

@@ -212,6 +212,10 @@ impl Window {
         unimplemented!()
     }
 
+    pub fn platform_window(&self) -> *mut libc::c_void {
+        self.window as *mut libc::c_void
+    }
+
     /// See the docs in the crate root file.
     pub fn get_api(&self) -> ::Api {
         ::Api::OpenGl