allow clip calls w/o op param, remove unnecessary kReplace ops

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2357333002

Review-Url: https://codereview.chromium.org/2357333002
diff --git a/gm/imagefilters.cpp b/gm/imagefilters.cpp
index 3e8d39a..e9f54a3 100644
--- a/gm/imagefilters.cpp
+++ b/gm/imagefilters.cpp
@@ -122,7 +122,7 @@
     for (int i = 0; i < count; ++i) {
         canvas->save();
         SkRRect rr = SkRRect::MakeRectXY(r.makeOffset(dx, dy), 20, 20);
-        canvas->clipRRect(rr, SkCanvas::kIntersect_Op, true);
+        canvas->clipRRect(rr, true);
         canvas->saveLayer({ &rr.getBounds(), nullptr, filters[i].get(), 0 });
         canvas->drawColor(0x40FFFFFF);
         canvas->restore();