Implement color filter as GrGLEffect

src/gpu/gl/GrGLSL.h

Index: src/gpu/gl/GrGLSL.h
diff --git a/src/gpu/gl/GrGLSL.h b/src/gpu/gl/GrGLSL.h
index b7457a2cbbb93c023790428be29aa7d582c801d4..828cda41bba82809a775cedca860a92100845280 100644
--- a/src/gpu/gl/GrGLSL.h
+++ b/src/gpu/gl/GrGLSL.h
@@ -197,6 +197,10 @@ public:
     static const char* zerosStr();
     static const char* onesStr();
 
+    bool isValid() const {
+        return kFullExpr_ExprType != fType || !fExpr.isEmpty();
+    }
+
 private:
     GrGLSLExpr(const char format[], const char in0[])
         : fType(kFullExpr_ExprType) {
@@ -217,10 +221,6 @@ private:
     template<int M>
     static GrGLSLExpr<N> VectorCast(const GrGLSLExpr<M>&);
 
-    bool isValid() const {
-        return kFullExpr_ExprType != fType || !fExpr.isEmpty();
-    }
-
     static const char* expandComponentStr();
     static const char* castStr();