Fix width calculation problems when maxWidth and minWidth are used

There are errors when using `minWidth` to determine the width of columns. Some columns are skipped and given a `drawnWidth` of undefined due to splicing elements from the array while iterating over it. Iterating backwards is the simplest fix for this (as far as I could tell).

As in, the problem was due to the following logic being used:

```
for(var i = 0; i < arr.length; i++){
    arr.splice(i, 1); // next iteration, when i is 1, we'll actually be looking at the element at position 2
}
```

Plunker showing the problem (minimally changed from tutorial 190): http://plnkr.co/edit/tSnDqn0gmBvoCJUpQnaX?p=preview

Plunker showing the fix: http://plnkr.co/edit/wZKuN2fFd30hVMhp0NeS?p=preview