Parallel cache - preliminary

include/core/SkAtomics.h

Index: include/core/SkAtomics.h
diff --git a/include/core/SkAtomics.h b/include/core/SkAtomics.h
index d31d9c64fb40ce13a20cc7db9a3e485bae0576d0..d03fc3235ca73e2276e0fa0e7b71369c996a44d2 100644
--- a/include/core/SkAtomics.h
+++ b/include/core/SkAtomics.h
@@ -58,6 +58,10 @@ public:
         return sk_atomic_fetch_add(&fVal, val, mo);
     }
 
+    T fetch_sub(const T& val, sk_memory_order mo = sk_memory_order_seq_cst) {
+        return sk_atomic_fetch_sub(&fVal, val, mo);
+    }
+
     bool compare_exchange(T* expected, const T& desired,
                           sk_memory_order success = sk_memory_order_seq_cst,
                           sk_memory_order failure = sk_memory_order_seq_cst) {