Add bigtileimagefilter GM & improvements to toString

src/effects/SkXfermodeImageFilter.cpp

Index: src/effects/SkXfermodeImageFilter.cpp
diff --git a/src/effects/SkXfermodeImageFilter.cpp b/src/effects/SkXfermodeImageFilter.cpp
index f98247a2f47c98f8f1f209b37f7921b76abb4d55..599fd42b12d0987bfa631f47d593d04f7b6296b2 100644
--- a/src/effects/SkXfermodeImageFilter.cpp
+++ b/src/effects/SkXfermodeImageFilter.cpp
@@ -105,7 +105,18 @@ void SkXfermodeImageFilter::toString(SkString* str) const {
     if (fMode) {
         fMode->toString(str);
     }
-    str->append("))");
+    str->append(")");
+    if (this->getInput(0)) {
+        str->appendf("foreground: (");
+        this->getInput(0)->toString(str);
+        str->appendf(")");
+    }
+    if (this->getInput(1)) {
+        str->appendf("background: (");
+        this->getInput(1)->toString(str);
+        str->appendf(")");
+    }
+    str->append(")");
 }
 #endif