Improve thin horiz/vertical dashed lines by not forcing width to 1.

It looks like a bug slipped in quite a while back here:
https://codereview.chromium.org/1092793006/diff/30002/src/gpu/effects/GrDashingEffect.cpp

where a width snapping that was supposed to apply to non-AA lines was
changed to apply to AA lines.

Adds GM that tests < 1 pixel wide dashed horiz/vertical lines.

The lines look better with the change but are still a little too thick,
depending on the subpixel offset.

BUG: chromium:1049028

Change-Id: I45734f5ef55548b62c188d9f55d3150c00059eed
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/288628
Reviewed-by: Greg Daniel <[email protected]>
Commit-Queue: Brian Salomon <[email protected]>

Author: bsalomon

gm/dashing.cpp

@@ -585,6 +585,23 @@ DEF_SIMPLE_GM(dash_line_zero_off_interval, canvas, 160, 330) {
     }
 }
 
+DEF_SIMPLE_GM(thin_aa_dash_lines, canvas, 110, 110) {
+    SkPaint paint;
+    static constexpr SkScalar kIntervals[] = {10, 5};
+    paint.setPathEffect(SkDashPathEffect::Make(kIntervals, SK_ARRAY_COUNT(kIntervals), 0.f));
+    paint.setAntiAlias(true);
+    paint.setStrokeWidth(0.25f);
+    // substep moves the subpixel offset every iteration.
+    static constexpr SkScalar kSubstep = 0.05f;
+    // We will draw a grid of horiz/vertical lines that pass through each other's off intervals.
+    static constexpr SkScalar kStep = kIntervals[0] + kIntervals[1];
+    canvas->translate(kIntervals[1], kIntervals[1]);
+    for (SkScalar x = -.5f*kIntervals[1]; x < 105; x += (kStep + kSubstep)) {
+        canvas->drawLine({x, 0}, {x, 100}, paint);
+        canvas->drawLine({0, x}, {100, x}, paint);
+    }
+}
+
 //////////////////////////////////////////////////////////////////////////////
 
 DEF_GM(return new DashingGM;)

src/gpu/ops/GrDashOp.cpp

@@ -513,7 +513,7 @@ class DashOp final : public GrMeshDrawOp {
             SkScalar devPhase = draw.fPhase * args.fParallelScale;
             SkScalar strokeWidth = args.fSrcStrokeWidth * args.fPerpendicularScale;
 
-            if ((strokeWidth < 1.f && useAA) || 0.f == strokeWidth) {
+            if ((strokeWidth < 1.f && !useAA) || 0.f == strokeWidth) {
                 strokeWidth = 1.f;
             }