Fix rounded corner rendering

[Long0x0]

test case:

use floem::{peniko::Color, view::View, views::Decorators, widgets::button};

fn app_view() -> impl View {
    button(|| "text").style(|s| {
        s.border_radius(15.0)
            .border(20.0)
            .width(200.0)
            .height(100.0)
            .margin(100.0)
            .background(Color::RED)
            .border_color(Color::BLUE)
    })
}

fn main() {
    floem::launch(app_view);
}

before:

after:

It still won't work if border_radius < border_width / 2, since the radius is limited by the stroke width.