Separate CPU driver impl from reference driver.

Change-Id: Ifb484edda665959b81d7b1f890d108bfa20a535d
diff --git a/rsScript.h b/rsScript.h
index 6339f49..8afd3bc 100644
--- a/rsScript.h
+++ b/rsScript.h
@@ -72,7 +72,6 @@
             char const **exportedPragmaValueList;
 
             int (* root)();
-            bool isThreadable;
         };
         DriverInfo info;
     };
@@ -85,7 +84,7 @@
 
     struct Enviroment_t {
         int64_t mStartTimeMillis;
-        int64_t mLastDtTime;
+        mutable int64_t mLastDtTime;
 
         ObjectBaseRef<ProgramVertex> mVertex;
         ObjectBaseRef<ProgramFragment> mFragment;