throttle function implements as debounce

https://github.com/impress/impress.js/blob/master/src/lib/util.js#L65-L74

```javascript
var throttle = function( fn, delay ) {
var timer = null;
return function() {
var context = this, args = arguments;
window.clearTimeout( timer );
timer = window.setTimeout( function() {
fn.apply( context, args );
}, delay );
};
};
```

Start with src/lib/util.js lines 65-74 and inspect how the throttle utility is used elsewhere in the project. Compare the reported behavior with the expected throttle behavior, then verify that the utility no longer acts like a debounce and that existing callers still work correctly.

Written by the indexing model from the issue text.