Improve the curve tessellation subdivision of the path renderers.

The stencil-and-cover and tessellating path renderers currently
tessellate curves to linear segments to within one screen space pixel.
This is fairly poor accuracy, as can be seen from the strokecircle GM:
there are noticeable rotating bumps on the circles.

Improving the tolerance to 0.25 pixel eliminates the bumps, and
approximates the 16x supersampling of the raster path.
This does have a performance hit: 3-6% on desktop on
the IE chalkboard demo, ~1% on Nexus 7 2013.

Note: this will require rebaselining a number of GPU and  MSAA images
in Skia, but nothing in Chrome (yet).

BUG=skia:3731

Review URL: https://codereview.chromium.org/1072273007

1 files changed, 7 insertions, 0 deletions

diff --git a/src/gpu/GrPathUtils.h b/src/gpu/GrPathUtils.h
index c15fd7ae10..250b0a2324 100644
--- a/src/gpu/GrPathUtils.h
+++ b/src/gpu/GrPathUtils.h
@@ -168,5 +168,12 @@ namespace GrPathUtils {
     // If you transform the points the lines will also need to be transformed. This can be done
     // by mapping the lines with the inverse-transpose of the matrix used to map the points.
     void getCubicKLM(const SkPoint p[4], SkScalar klm[9]);
+
+    // When tessellating curved paths into linear segments, this defines the maximum distance
+    // in screen space which a segment may deviate from the mathmatically correct value.
+    // Above this value, the segment will be subdivided.
+    // This value was chosen to approximate the supersampling accuracy of the raster path (16
+    // samples, or one quarter pixel).
+    static const SkScalar kDefaultTolerance = SkDoubleToScalar(0.25);
 };
 #endif