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.

crates/eframe/src/web/mod.rs

@@ -178,10 +178,8 @@ fn canvas_size_in_points(canvas: &web_sys::HtmlCanvasElement, ctx: &egui::Contex
 // ----------------------------------------------------------------------------
 
 /// Set the cursor icon.
-fn set_cursor_icon(cursor: egui::CursorIcon) -> Option<()> {
-    let document = web_sys::window()?.document()?;
-    document
-        .body()?
+fn set_cursor_icon(canvas: &web_sys::HtmlCanvasElement, cursor: egui::CursorIcon) -> Option<()> {
+    canvas
         .style()
         .set_property("cursor", cursor_web_name(cursor))
         .ok()