Add a function to convert a texture backed SkImage to raster.

src/image/SkImage.cpp

Index: src/image/SkImage.cpp
diff --git a/src/image/SkImage.cpp b/src/image/SkImage.cpp
index 88cf779081bf47ed4d10619904db6472c6a74d72..b7f4225633f831de7ba03e6b9f258b5ec8b11042 100644
--- a/src/image/SkImage.cpp
+++ b/src/image/SkImage.cpp
@@ -431,6 +431,10 @@ sk_sp<SkImage> SkImage::makeTextureImage(GrContext*) const {
     return nullptr;
 }
 
+sk_sp<SkImage> SkImage::makeNonTextureImage() const {
+    return sk_ref_sp(const_cast<SkImage*>(this));
+}
+
 #endif
 
 ///////////////////////////////////////////////////////////////////////////////////////////////////