bytebeat-render.js: define math function exp

js-programming/bytebeat-render.js

@@ -53,6 +53,7 @@ var atanh = Math.atanh
 var cbrt = Math.cbrt
 var cos = Math.cos
 var cosh = Math.cosh
+var exp = Math.exp
 var floor = Math.floor
 var log = Math.log
 var log2 = Math.log2