Convert SkPicture's generation ID to a unique ID

include/core/SkTypes.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkTypes_DEFINED
 #define SkTypes_DEFINED
 
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 /** maximum representable milliseconds
 */
 #define SK_MSecMax 0x7FFFFFFF
 /** Returns a < b for milliseconds, correctly handling wrap-around from 0xFFFFFFFF to 0
 */
 #define SkMSec_LT(a, b)     ((int32_t)(a) - (int32_t)(b) < 0)
 /** Returns a <= b for milliseconds, correctly handling wrap-around from 0xFFFFFFFF to 0
 */
 #define SkMSec_LE(a, b)     ((int32_t)(a) - (int32_t)(b) <= 0)
 
+/** The generation IDs in Skia reserve 0 has an invalid marker.
+ */
+#define SK_InvalidGenID     0
+
 /****************************************************************************
     The rest of these only build with C++
 */
 #ifdef __cplusplus
 
 /** Faster than SkToBool for integral conditions. Returns 0 or 1
 */
 static inline int Sk32ToBool(uint32_t n) {
     return (n | (0-n)) >> 31;
 }
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 private:
     void*       fPtr;
     size_t      fSize;  // can be larger than the requested size (see kReuse)
     uint32_t    fStorage[(kSize + 3) >> 2];
 };
 // Can't guard the constructor because it's a template class.
 
 #endif /* C++ */
 
 #endif