allow canvas to force conservative clips (for speed)

src/core/SkDeviceLooper.cpp

Index: src/core/SkDeviceLooper.cpp
diff --git a/src/core/SkDeviceLooper.cpp b/src/core/SkDeviceLooper.cpp
index 9346465dfc9f28f102bcdb1903819f8bf25b4578..a8350cc0272c9f2a278ab3ff50fea96ebeca2f50 100644
--- a/src/core/SkDeviceLooper.cpp
+++ b/src/core/SkDeviceLooper.cpp
@@ -10,9 +10,10 @@
 SkDeviceLooper::SkDeviceLooper(const SkBitmap& base,
                                const SkRasterClip& rc,
                                const SkIRect& bounds, bool aa)
-: fBaseBitmap(base)
-, fBaseRC(rc)
-, fDelta(aa ? kAA_Delta : kBW_Delta)
+    : fBaseBitmap(base)
+    , fBaseRC(rc)
+    , fSubsetRC(rc.isForceConservativeRects())
+    , fDelta(aa ? kAA_Delta : kBW_Delta)
 {
     // sentinels that next() has not yet been called, and so our mapper functions
     // should not be called either.