have canvas send discard instead of retain if the draw would overwrite everything

src/image/SkSurface.cpp

Index: src/image/SkSurface.cpp
diff --git a/src/image/SkSurface.cpp b/src/image/SkSurface.cpp
index 2e8c7fa87ebe280a72547d24b12201630b41eec3..bfc019bd4fa7c2a6f7711689ef346e35a663ad22 100644
--- a/src/image/SkSurface.cpp
+++ b/src/image/SkSurface.cpp
@@ -86,6 +86,10 @@ void SkSurface_Base::onDraw(SkCanvas* canvas, SkScalar x, SkScalar y, const SkPa
     }
 }
 
+bool SkSurface_Base::outstandingImageSnapshot() const {
+    return fCachedImage && !fCachedImage->unique();
+}
+
 void SkSurface_Base::aboutToDraw(ContentChangeMode mode) {
     this->dirtyGenerationID();