Add default version of fsDistanceVector if it is required but not implemented

src/gpu/glsl/GrGLSLProgramBuilder.cpp

Index: src/gpu/glsl/GrGLSLProgramBuilder.cpp
diff --git a/src/gpu/glsl/GrGLSLProgramBuilder.cpp b/src/gpu/glsl/GrGLSLProgramBuilder.cpp
index a6bff8a85ac983fadd717d80f17ee5e4f19bc068..f32042d821810eb0bdafb347d55d087636b75bcf 100644
--- a/src/gpu/glsl/GrGLSLProgramBuilder.cpp
+++ b/src/gpu/glsl/GrGLSLProgramBuilder.cpp
@@ -88,11 +88,17 @@ void GrGLSLProgramBuilder::emitAndInstallPrimProc(const GrPrimitiveProcessor& pr
     this->nameExpression(outputCoverage, "outputCoverage");
 
     const char* distanceVectorName = nullptr;
-    if (this->fPipeline.usesDistanceVectorField() && proc.implementsDistanceVector()) {
+    if (this->fPipeline.usesDistanceVectorField()) {
         distanceVectorName = fFS.distanceVectorName();
         fFS.codeAppend( "// Normalized vector to the closest geometric edge (in device space)\n");
         fFS.codeAppend( "// Distance to the edge encoded in the z-component\n");
-        fFS.codeAppendf("vec4 %s;", distanceVectorName);
+
+        if (proc.implementsDistanceVector()) {
+            fFS.codeAppendf("vec4 %s;", distanceVectorName);
+        } else {
+            fFS.codeAppendf("vec4 %s = vec4(1,0,0,0); // not implemented by primitive processor\n",
+                            distanceVectorName);
+        }
     }
 
     // Enclose custom code in a block to avoid namespace conflicts