eachActiveFeature() to return only layers currently in view

[jgravois]

resolves #933.

not only did _currentSnapshot not already isolate features currently in view, i found that it just was just growing in size as duplicate keys were appended throughout the browser session.

@keithpower i'd be curious to see how the approach i'm proposing here compares to your own technique if you can spare the time to take a look.

[keithpower]

Thanks @jgravois , good catch on the duplicate keys. Your check for duplicates wasn't catching them in my testing. I used this instead and it appears to work. Note, the this_cache was also getting duplicates

if (-1 === this._currentSnapshot.indexOf(id)) {
    this._currentSnapshot.push(id);
}
if (-1 === this._cache[key].indexOf(id)) {
    this._cache[key].push(id);
}

The eachActiveFeature is very similar to how I implemented it on the application level. I will switch to this once I get time. I used contains instead of intersect because I was only dealing with points. I tihnk you'll want the check for the currentSnapshot in this function too. If a where is set at some point then this function will return features that may have been filtered out by the where. So it could look like this instead.

 eachActiveFeature: function (fn, context) {
     // figure out roughly which layers are in view
     if (this._map) {
       var activeBounds = this._map.getBounds();
       for (var i in this._layers) {
         if (activeBounds.intersects(this._layers[i].getBounds()) &&  -1 !== this._currentSnapshot.indexOf(this._layers[i].feature.id)) {
           fn.call(context, this._layers[i]);
         }
       }
     }
     return this;
   },

I have some other potential improvements around the whole setWhere functionality that I will create a pull request for in the next week or so once I get past some deadlines.

[jgravois]

@keithpower much obliged!