Set clip_rect_margin to zero by default (#4357)

### What

Fix a minor visual glitch due to
[`clip_rect_margin`](https://docs.rs/egui/latest/egui/style/struct.Visuals.html#structfield.clip_rect_margin)'s
default value of 3.0px.

Before:
<img width="271" alt="image"
src="https://github.com/rerun-io/rerun/assets/49431240/e0596797-8907-4e3e-8706-db0490768f01">

After:

<img width="273" alt="image"
src="https://github.com/rerun-io/rerun/assets/49431240/3f77f3b2-2cb2-454e-9366-f6ae9ea1e438">


### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested [app.rerun.io](https://app.rerun.io/pr/4357) (if
applicable)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG

- [PR Build Summary](https://build.rerun.io/pr/4357)
- [Docs
preview](https://rerun.io/preview/267953d399716d700c978a6db7c183f7d45e94d9/docs)
<!--DOCS-PREVIEW-->
- [Examples
preview](https://rerun.io/preview/267953d399716d700c978a6db7c183f7d45e94d9/examples)
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- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
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crates/re_ui/src/design_tokens.rs

@@ -171,6 +171,9 @@ fn apply_design_tokens(ctx: &egui::Context) -> DesignTokens {
     egui_style.spacing.item_spacing = egui::vec2(8.0, 8.0);
     egui_style.spacing.menu_margin = crate::ReUi::view_padding().into();
 
+    // avoid some visual glitches with the default non-zero value
+    egui_style.visuals.clip_rect_margin = 0.0;
+
     // Add stripes to grids and tables?
     egui_style.visuals.striped = false;
     egui_style.visuals.indent_has_left_vline = false;