Adding an option to render only the shadow in SkDropShadowImageFilter

gm/imagefiltersbase.cpp

Index: gm/imagefiltersbase.cpp
diff --git a/gm/imagefiltersbase.cpp b/gm/imagefiltersbase.cpp
index d1bd5a72df80c1cafdae882ba79f44c9815c91b9..4546b8add2486a5e17545d0f05be05b9bbd74b60 100644
--- a/gm/imagefiltersbase.cpp
+++ b/gm/imagefiltersbase.cpp
@@ -231,7 +231,8 @@ protected:
             FailImageFilter::Create(),
             SkColorFilterImageFilter::Create(cf),
             SkBlurImageFilter::Create(12.0f, 0.0f),
-            SkDropShadowImageFilter::Create(10.0f, 5.0f, 3.0f, 3.0f, SK_ColorBLUE),
+            SkDropShadowImageFilter::Create(10.0f, 5.0f, 3.0f, 3.0f, SK_ColorBLUE,
+                SkDropShadowImageFilter::kDrawShadowAndForeground_ShadowMode),
         };
         cf->unref();