In a small saucepan, whisk together granulated sugar and cornstarch until well blended. Stir in cold water, lime juice and raspberries. Cook over medium heat whisking frequently, until mixture reaches a boil. Reduce to a simmer and cook about 5 minutes whisking frequently, until sauce has thickened. Remove from heat and force sauce through a fine mesh strainer (to remove seeds) into a bowl or airtight container. Cover and refrigerate.