add debugging wrapper blitter to ensure unclipped scan-conversion is safe

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4629

Change-Id: Ie6c35b28a6e8e87bf230e50f9940d3924d12969a
Reviewed-on: https://skia-review.googlesource.com/4629
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Yuqian Li <liyuqian@google.com>
diff --git a/src/core/SkScanPriv.h b/src/core/SkScanPriv.h
index 798cae6..4ff91bb 100644
--- a/src/core/SkScanPriv.h
+++ b/src/core/SkScanPriv.h
@@ -23,6 +23,9 @@
 private:
     SkRectClipBlitter   fRectBlitter;
     SkRgnClipBlitter    fRgnBlitter;
+#ifdef SK_DEBUG
+    SkRectClipCheckBlitter fRectClipCheckBlitter;
+#endif
     SkBlitter*          fBlitter;
     const SkIRect*      fClipRect;
 };