Faster dashed line generation

[lampsitter]

The Shape::dashed_line was quite hot in my program, so I decided to optimize it.
In addition to the original function I added an alternative function, which allows reuse of an existing Vec.

Old:

running 1 test
test single ... bench:      81,939 ns/iter (+/- 7,938)

New:

running 2 tests
test single ... bench:      19,690 ns/iter (+/- 1,336)
test many   ... bench:      10,081 ns/iter (+/- 347)

Benchmark code:

#![feature(test)]

extern crate test;
use eframe::egui::{Shape, Stroke};
use test::Bencher;

#[bench]
fn single(b: &mut Bencher) {
    b.iter(|| {
        let mut v = Vec::new();
        use eframe::egui;

        let line = [
            egui::pos2(0.0, 0.0),
            egui::pos2(50.0, 0.0),
            egui::pos2(100.0, 1.0),
        ];

        for _ in 0..100 {
            v.extend(Shape::dashed_line(
                &line,
                Stroke::new(1.5, eframe::egui::Color32::RED),
                10.0,
                2.5,
            ));
        }

        test::black_box(v);
    });
}

// Must be commented out when run against master
#[bench]
fn many(b: &mut Bencher) {
    b.iter(|| {
        let mut v = Vec::new();
        use eframe::egui;

        let line = [
            egui::pos2(0.0, 0.0),
            egui::pos2(50.0, 0.0),
            egui::pos2(100.0, 1.0),
        ];

        for _ in 0..100 {
            Shape::dashed_line_many(
                &line,
                Stroke::new(1.5, eframe::egui::Color32::RED),
                10.0,
                2.5,
                &mut v,
            );
        }

        test::black_box(v);
    });
}

[emilk]

Great! Feel free to also add the benchmark, perhaps to a new epaint/benches/benchmark.rs file?

[emilk]

thanks!