Make GrGLShaderBuilder::fragmentPosition() return a vec4, with 1.0 as the zw components when in the…

src/gpu/effects/GrDitherEffect.cpp

Index: src/gpu/effects/GrDitherEffect.cpp
diff --git a/src/gpu/effects/GrDitherEffect.cpp b/src/gpu/effects/GrDitherEffect.cpp
index e82b565b5ecc1a7bf23aa3c8bafbef41b8393540..7409e5ff7151818442b74ed68938d27ec38cd9b0 100644
--- a/src/gpu/effects/GrDitherEffect.cpp
+++ b/src/gpu/effects/GrDitherEffect.cpp
@@ -102,7 +102,7 @@ void GLDitherEffect::emitCode(GrGLShaderBuilder* builder,
     // For each channel c, add the random offset to the pixel to either bump
     // it up or let it remain constant during quantization.
     builder->fsCodeAppendf("\t\tfloat r = "
-                           "fract(sin(dot(%s ,vec2(12.9898,78.233))) * 43758.5453);\n",
+                           "fract(sin(dot(%s.xy ,vec2(12.9898,78.233))) * 43758.5453);\n",
                            builder->fragmentPosition());
     builder->fsCodeAppendf("\t\t%s = (1.0/255.0) * vec4(r, r, r, r) + %s;\n",
                            outputColor, GrGLSLExpr4(inputColor).c_str());