Only apply cursor style to the <canvas> (#8036)

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This improves cases where the canvas does not cover the full screen,
which was a goal in [release
0.28.0](https://github.com/emilk/egui/releases/tag/0.28.0).

* Closes <https://github.com/emilk/egui/issues/8035>
* [X] I have followed the instructions in the PR template

crates/eframe/src/web/app_runner.rs

@@ -393,7 +393,7 @@ impl AppRunner {
             }
         }
 
-        super::set_cursor_icon(cursor_icon);
+        super::set_cursor_icon(self.canvas(), cursor_icon);
 
         if self.has_focus() {
             // The eframe app has focus.