When a surface is backed by an external render target force a copy on image snap

src/image/SkSurface_Gpu.cpp

Index: src/image/SkSurface_Gpu.cpp
diff --git a/src/image/SkSurface_Gpu.cpp b/src/image/SkSurface_Gpu.cpp
index 5345ceee204bd28c8392ba5dec898c7f6bcc9042..173a0d81d7135929efd0e8bdde662480b4621671 100644
--- a/src/image/SkSurface_Gpu.cpp
+++ b/src/image/SkSurface_Gpu.cpp
@@ -82,8 +82,10 @@ SkImage* SkSurface_Gpu::onNewImageSnapshot(SkBudgeted budgeted, ForceCopyMode fo
     SkASSERT(rt);
     GrTexture* tex = rt->asTexture();
     SkAutoTUnref<GrTexture> copy;
-    // TODO: Force a copy when the rt is an external resource.
-    if (kYes_ForceCopyMode == forceCopyMode || !tex) {
+    // If the original render target is a buffer originally created by the client, then we don't
+    // want to ever retarget the SkSurface at another buffer we create. Force a copy now to avoid
+    // copy-on-write.
+    if (kYes_ForceCopyMode == forceCopyMode || !tex || rt->resourcePriv().isExternal()) {
         GrSurfaceDesc desc = fDevice->accessRenderTarget()->desc();
         GrContext* ctx = fDevice->context();
         desc.fFlags = desc.fFlags & ~kRenderTarget_GrSurfaceFlag;