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

src/gpu/GrAARectRenderer.cpp

Index: src/gpu/GrAARectRenderer.cpp
diff --git a/src/gpu/GrAARectRenderer.cpp b/src/gpu/GrAARectRenderer.cpp
index b5d2db9246b6a756f589536cec40106a7fcccffb..497b3cfbb8d6b11903a5b9d741596f5e2e08081d 100644
--- a/src/gpu/GrAARectRenderer.cpp
+++ b/src/gpu/GrAARectRenderer.cpp
@@ -200,7 +200,7 @@ public:
             builder->fsCodeAppend("\tfloat scaleH = min(1.0, 2.0*insetH/spanH);\n");
 
             // Compute the coverage for the rect's width
-            builder->fsCodeAppendf("\tvec2 offset = %s - %s.xy;\n",
+            builder->fsCodeAppendf("\tvec2 offset = %s.xy - %s.xy;\n",
                                    builder->fragmentPosition(), fsRectEdgeName);
             builder->fsCodeAppendf("\tfloat perpDot = abs(offset.x * %s.w - offset.y * %s.z);\n",
                                    fsRectEdgeName, fsRectEdgeName);