Correct UPC and EAN rendering

Made corrections to the rendering of UPC and EAN symbols in render.c
including taking account of EAN-2 and EAN-5 addon codes, correct text sizing
and simplified guard bar extension.

backend/library.c

@@ -33,7 +33,7 @@
 struct zint_symbol *ZBarcode_Create()
 {
 	struct zint_symbol *symbol;
-	int i, j;
+	int i;
 	
 	symbol = (struct zint_symbol*)malloc(sizeof(*symbol));
 	if (!symbol) return NULL;