replace_import = function(v)
	{
		var token = v.split("\"");
		var id = token[1];
		if( already_imported[id] )
			return "//already imported: " + id + "\n";
		var file = files[id];
		already_imported[ id ] = true;
		if(file)
			return file + "\n";
		return "//import code not found: " + id + "\n";
	}
Given a source code with the directive #import it expands it inserting the code using Shader.files to fetch for import files.
Warning: Imports are evaluated only the first inclusion, the rest are ignored to avoid double inclusion of functions
         Also, imports cannot have other imports inside.
@method Shader.expandImports
@param {String} code the source code
@param {Object} files [Optional] object with files to import from (otherwise Shader.files is used)
@return {String} the code with the lines #import removed and replaced by the code