fix path op conic bugs

src/pathops/SkPathOpsConic.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 #include "SkIntersections.h"
 #include "SkLineParameters.h"
 #include "SkPathOpsConic.h"
 #include "SkPathOpsCubic.h"
@@ skipping 135 matching lines @@
         cy = conic_eval_numerator(&fPts[0].fY, fWeight, t2);
         cz = conic_eval_denominator(fWeight, t2);
     } else {
         cx = fPts[0].fX;
         cy = fPts[0].fY;
         cz = 1;
     }
     double bx = 2 * dx - (ax + cx) / 2;
     double by = 2 * dy - (ay + cy) / 2;
     double bz = 2 * dz - (az + cz) / 2;
-    SkDConic dst = {{{{ax / az, ay / az}, {bx / bz, by / bz}, {cx / cz, cy / cz}}}, 0 };
-    SkDPoint dMidAC = { (dst.fPts[0].fX + dst.fPts[2].fX) / 2,
-                        (dst.fPts[0].fY + dst.fPts[2].fY) / 2 };
-    SkDPoint dMid = { dx / dz, dy / dz };
-    SkDVector dWNumer = dMidAC - dMid;
-    SkDVector dWDenom = dMid - dst.fPts[1];
-    dst.fWeight = dWNumer.length() / dWDenom.length();
+    SkDConic dst = {{{{ax / az, ay / az}, {bx / bz, by / bz}, {cx / cz, cy / cz}}},
+            SkDoubleToScalar(bz / sqrt(az * cz)) };
     return dst;
 }
 
 SkDPoint SkDConic::subDivide(const SkDPoint& a, const SkDPoint& c, double t1, double t2,
         SkScalar* weight) const {
     SkDConic chopped = this->subDivide(t1, t2);
     *weight = chopped.fWeight;
     return chopped[1];
 }