incorect z-index sorting fixed

If you omit compare function in javascript `sort()` method it will sort  lexicographically (in dictionary order) according to the string conversion of each element. 

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort#Parameters

For example 
[0, 9, 8, 10].sort()
will place element with z-index 10 on 2nd position of the array
Merge pull request #297 from Grom-S/patch-1

incorect z-index sorting fixed