egui_demo_app: don't do a transparent native window

It doesn't work great on Mac

egui_demo_app/src/main.rs

@@ -8,8 +8,6 @@ fn main() {
     tracing_subscriber::fmt::init();
 
     let options = eframe::NativeOptions {
-        // Let's show off that we support transparent windows
-        transparent: true,
         drag_and_drop_support: true,
         ..Default::default()
     };