Avoid re-rendering stencil clip for every draw with reducable clip stack

src/gpu/GrClipMaskCache.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrClipMaskCache_DEFINED
 #define GrClipMaskCache_DEFINED
 
@@ skipping 18 matching lines @@
             temp->~GrClipStackFrame();
             fStack.pop_back();
         }
     }
 
     bool canReuse(int32_t clipGenID, const SkIRect& bounds) {
 
         SkASSERT(clipGenID != SkClipStack::kWideOpenGenID);
         SkASSERT(clipGenID != SkClipStack::kEmptyGenID);
 
-        if (SkClipStack::kInvalidGenID == clipGenID) {
-            return false;
-        }
-
         GrClipStackFrame* back = (GrClipStackFrame*) fStack.back();
 
         // We could reuse the mask if bounds is a subset of last bounds. We'd have to communicate
         // an offset to the caller.
         if (back->fLastMask.texture() &&
             back->fLastBound == bounds &&
             back->fLastClipGenID == clipGenID) {
             return true;
         }
 
@@ skipping 177 matching lines @@
         SkIRect                 fLastBound;
     };
 
     GrContext*   fContext;
     SkDeque      fStack;
 
     typedef SkNoncopyable INHERITED;
 };
 
 #endif // GrClipMaskCache_DEFINED