Fix SkClipStack bug with inverse-filled difference elements

src/core/SkClipStack.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkAtomics.h"
 #include "SkCanvas.h"
 #include "SkClipStack.h"
@@ skipping 219 matching lines @@
             // that aren't set are whatever isn't set in the previous
             // clip and whatever this clip carves out
             fFiniteBound.join(prevFinite);
             fFiniteBoundType = kInsideOut_BoundsType;
             break;
         case kPrev_InvCur_FillCombo:
             // In this case everything outside of this clip's bound
             // is erased, so the only pixels that can remain set
             // occur w/in the intersection of the two finite bounds
             if (!fFiniteBound.intersect(prevFinite)) {
-                this->setEmpty();
-            } else {
-                fFiniteBoundType = kNormal_BoundsType;
+                fFiniteBound.setEmpty();
+                fGenID = kEmptyGenID;
             }
+            fFiniteBoundType = kNormal_BoundsType;
             break;
         case kPrev_Cur_FillCombo:
             // The most conservative result bound is that of the
             // prior clip. This could be wildly incorrect if the
             // second clip either exactly matches the first clip
             // (which should yield the empty set) or reduces the
             // size of the prior bound (e.g., if the second clip
             // exactly matched the bottom half of the prior clip).
             // We ignore these two possibilities.
             fFiniteBound = prevFinite;
@@ skipping 657 matching lines @@
 
 void SkClipStack::dump() const {
     B2TIter iter(*this);
     const Element* e;
     while ((e = iter.next())) {
         e->dump();
         SkDebugf("\n");
     }
 }
 #endif