Fix elliptical rrect clip shaders for large radii on devices with mediump

src/gpu/effects/GrOvalEffect.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrOvalEffect.h"
 
 #include "GrFragmentProcessor.h"
@@ skipping 267 matching lines @@
 };
 
 GLEllipseEffect::GLEllipseEffect(const GrProcessor& effect) {
     fPrevRadii.fX = -1.f;
 }
 
 void GLEllipseEffect::emitCode(EmitArgs& args) {
     const EllipseEffect& ee = args.fFp.cast<EllipseEffect>();
     const char *ellipseName;
     // The ellipse uniform is (center.x, center.y, 1 / rx^2, 1 / ry^2)
+    // The last two terms can underflow on mediump, so we use highp.
     fEllipseUniform = args.fBuilder->addUniform(GrGLProgramBuilder::kFragment_Visibility,
-                                         kVec4f_GrSLType, kDefault_GrSLPrecision,
+                                         kVec4f_GrSLType, kHigh_GrSLPrecision,
                                          "ellipse",
                                          &ellipseName);
 
     GrGLFragmentBuilder* fsBuilder = args.fBuilder->getFragmentShaderBuilder();
     const char* fragmentPos = fsBuilder->fragmentPosition();
 
     // d is the offset to the ellipse center
     fsBuilder->codeAppendf("\t\tvec2 d = %s.xy - %s.xy;\n", fragmentPos, ellipseName);
     fsBuilder->codeAppendf("\t\tvec2 Z = d * %s.zw;\n", ellipseName);
     // implicit is the evaluation of (x/rx)^2 + (y/ry)^2 - 1.
@@ skipping 65 matching lines @@
         w /= 2;
         return CircleEffect::Create(edgeType, SkPoint::Make(oval.fLeft + w, oval.fTop + w), w);
     } else {
         w /= 2;
         h /= 2;
         return EllipseEffect::Create(edgeType, SkPoint::Make(oval.fLeft + w, oval.fTop + h), w, h);
     }
 
     return NULL;
 }