How to draw rectangle with round corner?

In `@gfx/canvas` there is a method called `arcTo`, which can help me drawing rounded triangle:
```ts
ctx.arcTo(350, 0, 350, 128, 50);
```
But seems that there is no similar method in this crate, how should I draw a rounded triangle with specific radius?

Start by reviewing the crate's path-drawing entry points and compare the requested behavior with @gfx/canvas's arcTo example. Done means documenting or enabling a way to draw a rounded triangle with a specified radius, with behavior that matches the issue's request.

Written by the indexing model from the issue text.