Added TextureUnit entry to osg::TransferFunction and osgTerrain::Layer

include/osg/TransferFunction

@@ -31,6 +31,13 @@ class OSG_EXPORT TransferFunction : public osg::Referenced
     
         TransferFunction();
 
+        /** Set the texture unit to assign layer to if required. 
+          * Negative values signifies that no texture unit has been assigned. */
+        void setTextureUnit(int textureUnit) { _textureUnit = textureUnit; }
+
+        /** Get the texture unit to assign layer to if required.*/
+        int getTextureUnit() const { return _textureUnit; }
+
         osg::Image* getImage() { return _image.get(); }
         const osg::Image* getImage() const { return _image.get(); }
         
@@ -45,7 +52,9 @@ class OSG_EXPORT TransferFunction : public osg::Referenced
         virtual ~TransferFunction();
 
         typedef std::vector<osg::Vec4> Colors;
-        Colors                     _colors;
+
+        int                         _textureUnit; 
+        Colors                      _colors;
         osg::ref_ptr<osg::Image>    _image;
         osg::ref_ptr<osg::Texture>  _texture;
         osg::ref_ptr<osg::Shader>   _shader;