Fix bug causing tooltips with dynamic content to shrink (#5168)

Affects `.on_hover_text(…)` with dynamic content (i.e. content that
changes over time).

* Closes https://github.com/emilk/egui/issues/5167

`.on_hover_ui` with dynamic content can still hit the shrinking problem.
The general solution depends on solving
https://github.com/emilk/egui/issues/5138 but a work-around is to add
this to your tooltips:

```diff
 response.on_hover_ui(|ui| {
+    ui.set_max_width(ui.spacing().tooltip_width);
     // …
 });
```

tests/test_size_pass/src/main.rs

@@ -103,6 +103,12 @@ fn main() -> eframe::Result {
                     ui.label("World");
                     ui.label("Hellooooooooooooooooooooooooo");
                 });
+
+            ui.separator();
+
+            let time = ui.input(|i| i.time);
+            ui.label("Hover for a tooltip with changing content")
+                .on_hover_text(format!("A number: {}", time % 10.0));
         });
     })
 }