use kFast_SrcRectConstraint to match drawAtlas semantics

BUG=skia:
TBR=

Review URL: https://codereview.chromium.org/1272743003
diff --git a/samplecode/SampleAtlas.cpp b/samplecode/SampleAtlas.cpp
index 0132b02..4cce487 100644
--- a/samplecode/SampleAtlas.cpp
+++ b/samplecode/SampleAtlas.cpp
@@ -33,7 +33,8 @@
         
         canvas->save();
         canvas->concat(matrix);
-        canvas->drawImageRect(atlas, &tex[i], tex[i].makeOffset(-tex[i].x(), -tex[i].y()), paint);
+        canvas->drawImageRect(atlas, &tex[i], tex[i].makeOffset(-tex[i].x(), -tex[i].y()), paint,
+                              SkCanvas::kFast_SrcRectConstraint);
         canvas->restore();
     }
 }