added initial GLSL support to skslc

src/sksl/ir/SkSLVariable.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
  
 #ifndef SKSL_VARIABLE
 #define SKSL_VARIABLE
 
@@ skipping 22 matching lines @@
     , fModifiers(modifiers)
     , fType(type)
     , fStorage(storage)
     , fIsReadFrom(false)
     , fIsWrittenTo(false) {}
 
     virtual std::string description() const override {
         return fModifiers.description() + fType.fName + " " + fName;
     }
 
-    const Modifiers fModifiers;
+    mutable Modifiers fModifiers;
     const Type& fType;
     const Storage fStorage;
 
     mutable bool fIsReadFrom;
     mutable bool fIsWrittenTo;
 
     typedef Symbol INHERITED;
 };
 
 } // namespace SkSL
 
 #endif