Use dark-light on Mac and Windows (#1726)

* Use dark-light on Mac and Windows

dark-light has a nasty problem on Linux: https://github.com/frewsxcv/rust-dark-light/issues/17

So we made dark-light opt-in in https://github.com/emilk/egui/pull/1437

This PR makes dark-light a default dependency again,
but only use it on Max and Windows.

This is controlled with the new NativeOptions::follow_system_theme.
If this isn't enabled, then NativeOptions::default_theme is used.

* Add eframe::WebOptions

egui_demo_app/src/lib.rs

@@ -34,5 +34,10 @@ pub fn start(canvas_id: &str) -> Result<(), wasm_bindgen::JsValue> {
     // Redirect tracing to console.log and friends:
     tracing_wasm::set_as_global_default();
 
-    eframe::start_web(canvas_id, Box::new(|cc| Box::new(WrapApp::new(cc))))
+    let web_options = eframe::WebOptions::default();
+    eframe::start_web(
+        canvas_id,
+        web_options,
+        Box::new(|cc| Box::new(WrapApp::new(cc))),
+    )
 }

egui_demo_app/src/wrap_app.rs

@@ -112,12 +112,6 @@ impl WrapApp {
             }
         }
 
-        if cc.integration_info.prefer_dark_mode == Some(false) {
-            cc.egui_ctx.set_visuals(egui::Visuals::light()); // use light mode if explicitly asked for
-        } else {
-            cc.egui_ctx.set_visuals(egui::Visuals::dark()); // use dark mode if there is no preference, or the preference is dark mode
-        }
-
         slf
     }