Determine the callback count while iterating through all layers

Previously, if a layer didn't have a `getFeature*ForPixel` type method, the loop count would be decremented.  This means that all layers would not be considered.  Instead, the callback count should be incremented before requests are made and decremented in the callback.

examples/select-features.js

@@ -47,9 +47,7 @@ var vector = new ol.layer.Vector({
   })
 });
 
-var select = new ol.interaction.Select({
-  layers: [vector]
-});
+var select = new ol.interaction.Select();
 
 var map = new ol.Map({
   interactions: ol.interaction.defaults().extend([select]),