From Art Trevs, add support for saving external shader files.

From Robert Osfield, adding missing member variable initializes and Output::getShaderFileNameForOutput() implementation

include/osgDB/WriteFile

@@ -105,6 +105,26 @@ inline bool writeNodeFile(const osg::Node& node, const std::string& filename)
     return writeNodeFile( node, filename, Registry::instance()->getOptions() );
 }
 
+/** Write an osg::Shader to file. 
+  * Return true on success,
+  * return false on failure.
+  * Use the Options object to control cache operations and file search paths in osgDB::Registry.
+  * The osgDB::Registry is used to load the appropriate ReaderWriter plugin
+  * for the filename extension, and this plugin then handles the request
+  * to write the specified file.*/
+extern OSGDB_EXPORT bool writeShaderFile(const osg::Shader& shader, const std::string& filename, const ReaderWriter::Options* options );
+
+/** Write an osg::Shader to file. 
+  * Return true on success,
+  * return false on failure.
+  * The osgDB::Registry is used to load the appropriate ReaderWriter plugin
+  * for the filename extension, and this plugin then handles the request
+  * to write the specified file.*/
+inline bool writeShaderFile(const osg::Shader& shader, const std::string& filename)
+{
+    return writeShaderFile( shader, filename, Registry::instance()->getOptions() );
+}
+
 }
 
 #endif