added initial GLSL support to skslc

src/sksl/SkSLCodeGenerator.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_CODEGENERATOR
 #define SKSL_CODEGENERATOR
 
 #include "ir/SkSLProgram.h"
 #include <vector>
 #include <iostream>
 
 namespace SkSL {
 
 /**
  * Abstract superclass of all code generators, which take a Program as input and produce code as
  * output.
  */
 class CodeGenerator {
 public:
-        virtual ~CodeGenerator() {}
-        
-    virtual void generateCode(Program& program, std::ostream& out) = 0;
+    virtual ~CodeGenerator() {}
+    
+    virtual void generateCode(const Program& program, std::ostream& out) = 0;
 };
 
 } // namespace
 
 #endif