gpui: Add paint_path example (#20499)

Release Notes:

- N/A

---

```
cargo run -p gpui --example painting
```

I added this demo to verify the detailed support of Path drawing in
GPUI.

Because of, when we actually used GPUI to draw a 2D line chart, we found
that the straight line Path#line_to did not support `anti-aliasing`, and
the drawn line looked very bad.

As shown in the demo image, if we zoom in on the image, we can clearly
see that all the lines are jagged.

I read and tried to make some appropriate adjustments to the functions
in Path, but since I have no experience in the graphics field, I still
cannot achieve anti-aliasing support so far.

I don't know if I used it wrong somewhere. I checked `curve_to` and
found that the curves drawn have anti-aliasing effects, as shown in the
arc part of the figure below.

<img width="1136" alt="image"
src="https://github.com/user-attachments/assets/4dfb7603-e746-43e9-b737-cff56b56329f">