Faster dashed line generation (#1027)

* Faster dashed lines generation

* Add dashed_line benchmark

epaint/benches/benchmark.rs

@@ -0,0 +1,43 @@
+use criterion::{black_box, criterion_group, criterion_main, Criterion};
+
+use epaint::{pos2, Color32, Shape, Stroke};
+
+fn single_dashed_lines(c: &mut Criterion) {
+    c.bench_function("single_dashed_lines", move |b| {
+        b.iter(|| {
+            let mut v = Vec::new();
+
+            let line = [pos2(0.0, 0.0), pos2(50.0, 0.0), pos2(100.0, 1.0)];
+
+            for _ in 0..100 {
+                v.extend(Shape::dashed_line(
+                    &line,
+                    Stroke::new(1.5, Color32::RED),
+                    10.0,
+                    2.5,
+                ));
+            }
+
+            black_box(v);
+        });
+    });
+}
+
+fn many_dashed_lines(c: &mut Criterion) {
+    c.bench_function("many_dashed_lines", move |b| {
+        b.iter(|| {
+            let mut v = Vec::new();
+
+            let line = [pos2(0.0, 0.0), pos2(50.0, 0.0), pos2(100.0, 1.0)];
+
+            for _ in 0..100 {
+                Shape::dashed_line_many(&line, Stroke::new(1.5, Color32::RED), 10.0, 2.5, &mut v);
+            }
+
+            black_box(v);
+        });
+    });
+}
+
+criterion_group!(benches, single_dashed_lines, many_dashed_lines);
+criterion_main!(benches);