Revert of Revert of Extract most of the mutable state of SkShader into a separate Context object.

src/core/SkSmallAllocator.h

 /*
  * Copyright 2014 Google, Inc
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkSmallAllocator_DEFINED
 #define SkSmallAllocator_DEFINED
 
@@ skipping 99 matching lines @@
             return NULL;
         }
         const size_t storageRemaining = SkAlign4(kTotalBytes) - fStorageUsed;
         storageRequired = SkAlign4(storageRequired);
         Rec* rec = &fRecs[fNumObjects];
         if (storageRequired > storageRemaining) {
             // Allocate on the heap. Ideally we want to avoid this situation,
             // but we're not sure we can catch all callers, so handle it but
             // assert false in debug mode.
             SkASSERT(false);
+            rec->fStorageSize = 0;
             rec->fHeapStorage = sk_malloc_throw(storageRequired);
             rec->fObj = static_cast<void*>(rec->fHeapStorage);
         } else {
             // There is space in fStorage.
+            rec->fStorageSize = storageRequired;
             rec->fHeapStorage = NULL;
             SkASSERT(SkIsAlign4(fStorageUsed));
             rec->fObj = static_cast<void*>(fStorage + (fStorageUsed / 4));
             fStorageUsed += storageRequired;
         }
         rec->fKillProc = destroyT<T>;
         fNumObjects++;
         return rec->fObj;
     }
 
+    /*
+     *  Free the memory reserved last without calling the destructor.
+     *  Can be used in a nested way, i.e. after reserving A and B, calling
+     *  freeLast once will free B and calling it again will free A.
+     */
+    void freeLast() {
+        SkASSERT(fNumObjects > 0);
+        Rec* rec = &fRecs[fNumObjects - 1];
+        sk_free(rec->fHeapStorage);
+        fStorageUsed -= rec->fStorageSize;
+
+        fNumObjects--;
+    }
+
 private:
     struct Rec {
-        void* fObj;
-        void* fHeapStorage;
-        void  (*fKillProc)(void*);
+        size_t fStorageSize;  // 0 if allocated on heap
+        void*  fObj;
+        void*  fHeapStorage;
+        void   (*fKillProc)(void*);
     };
 
     // Number of bytes used so far.
     size_t              fStorageUsed;
     // Pad the storage size to be 4-byte aligned.
     uint32_t            fStorage[SkAlign4(kTotalBytes) >> 2];
     uint32_t            fNumObjects;
     Rec                 fRecs[kMaxObjects];
 };
 
 #endif // SkSmallAllocator_DEFINED