Refine stroke expansion

Wire up regularization in parallel curve generation (now merged into branch). Fix zero length segment detection (it was overly eager, also triggering on degenerate tangents).

In response to review comments.

src/stroke.rs

@@ -227,7 +227,7 @@ fn stroke_undashed(
                 }
             }
             PathEl::QuadTo(p1, p2) => {
-                if p1 != p0 && p2 != p0 {
+                if p1 != p0 || p2 != p0 {
                     let q = QuadBez::new(p0, p1, p2);
                     let (tan0, tan1) = PathSeg::Quad(q).tangents();
                     ctx.do_join(style, tan0);
@@ -236,7 +236,7 @@ fn stroke_undashed(
                 }
             }
             PathEl::CurveTo(p1, p2, p3) => {
-                if p1 != p0 && p2 != p0 && p3 != p0 {
+                if p1 != p0 || p2 != p0 || p3 != p0 {
                     let c = CubicBez::new(p0, p1, p2, p3);
                     let (tan0, tan1) = PathSeg::Cubic(c).tangents();
                     ctx.do_join(style, tan0);
@@ -420,7 +420,11 @@ impl StrokeCtx {
         let co = CubicOffset::new(c, -0.5 * style.width);
         let forward = fit_with_opts(&co, tolerance, opts);
         self.forward_path.extend(forward.into_iter().skip(1));
-        let co = CubicOffset::new(c, 0.5 * style.width);
+        // A tuning parameter for regularization. A value too large may distort the curve,
+        // while a value too small may fail to generate smooth curves. This is a somewhat
+        // arbitrary value, and should be revisited.
+        const DIM_TUNE: f64 = 0.25;
+        let co = CubicOffset::new_regularized(c, 0.5 * style.width, tolerance * DIM_TUNE);
         let backward = fit_with_opts(&co, tolerance, opts);
         self.backward_path.extend(backward.into_iter().skip(1));
         self.last_pt = c.p3;