text input selection radius (#24307)

# Objective

Allow rounded corners for text input selection regions.

Fixes #23952

## Solution

* Added a `selection_radius` field to `TextCursorStyle`, defaults to
`0.`.
* In `extract_text_cursor` set a corner radius for each selection rect,
after clamping for short and inner corners.

---

Left rounded inner corners for a follow up, as they are significantly
more complicated to implement.

## Testing

Added an option to set the selection radius to the
`multiline_text_input` example:

```
cargo run --example multiline_text_input --features="system_clipboard"
```

<img width="400" alt="image"
src="https://github.com/user-attachments/assets/a10bb552-ac03-4e06-a287-53c57e5c2f45"
/>

## Showcase

<img width="400" alt="rounded"
src="https://github.com/user-attachments/assets/79d59c66-7fb0-4ad4-8a17-308c755c1f23"
/>
<img width="400" alt="r2"
src="https://github.com/user-attachments/assets/a3dfd8a6-f85f-4534-ba61-4a8c74e9793d"
/>
<img width="400" alt="r3"
src="https://github.com/user-attachments/assets/78e4d863-198e-4ee0-ad33-22e7cde7016a"
/>