Make copySurface work for texture dsts, return a bool, & add unit test.

include/gpu/GrDrawContext.h

Index: include/gpu/GrDrawContext.h
diff --git a/include/gpu/GrDrawContext.h b/include/gpu/GrDrawContext.h
index 9276e2b21ff147e16ad69cc6f93d951d5c776433..38e3a0b5479e6b2eb56f900c057700350d7055a5 100644
--- a/include/gpu/GrDrawContext.h
+++ b/include/gpu/GrDrawContext.h
@@ -47,7 +47,7 @@ class SK_API GrDrawContext : public SkRefCnt {
 public:
     ~GrDrawContext() override;
 
-    void copySurface(GrSurface* src, const SkIRect& srcRect, const SkIPoint& dstPoint);
+    bool copySurface(GrSurface* src, const SkIRect& srcRect, const SkIPoint& dstPoint);
 
     // TODO: it is odd that we need both the SkPaint in the following 3 methods.
     // We should extract the text parameters from SkPaint and pass them separately