Make GrGLEffects use an interface to append their code.

src/core/SkString.cpp

Index: src/core/SkString.cpp
===================================================================
--- src/core/SkString.cpp	(revision 8005)
+++ src/core/SkString.cpp	(working copy)
@@ -531,6 +531,13 @@
     this->append(buffer, strlen(buffer));
 }
 
+void SkString::appendf(const char format[], va_list args) {
+    char    buffer[kBufferSize];
+    VSNPRINTF(buffer, kBufferSize, format, args);
+
+    this->append(buffer, strlen(buffer));
+}
+
 void SkString::prependf(const char format[], ...) {
     char    buffer[kBufferSize];
     ARGS_TO_BUFFER(format, buffer, kBufferSize);