Add ViewportInfo::occluded and visible (#7948)

* Part of https://github.com/emilk/egui/issues/5112
* Part of https://github.com/emilk/egui/issues/5113
* Part of https://github.com/emilk/egui/issues/5136

Once we support calling `App::logic` when an app is occluded or
minimized, it is useful to know that it is, in fact, occluded or
minimized.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

crates/eframe/src/web/backend.rs

@@ -31,11 +31,18 @@ impl WebInput {
             time: Some(super::now_sec()),
             ..self.raw.take()
         };
-        raw_input
+        let viewport = raw_input
             .viewports
             .entry(egui::ViewportId::ROOT)
-            .or_default()
-            .native_pixels_per_point = Some(super::native_pixels_per_point());
+            .or_default();
+        viewport.native_pixels_per_point = Some(super::native_pixels_per_point());
+
+        // A hidden browser tab is effectively occluded.
+        let hidden = web_sys::window()
+            .and_then(|w| w.document())
+            .is_some_and(|doc| doc.hidden());
+        viewport.occluded = Some(hidden);
+
         raw_input
     }