title: OpenLayers skip loading of vector features above a specific resolution
body: <p>I am using OpenLayers to read vector features from GeoServer with a custom loader function on the source.
  Since there are a lot of features, I would like to skip loading them above a specific resolution, for example 20.
  The loader function receives the resolution as a parameter, so I can simply return without any xhr loading. It seems fine until this point.
  The problem is that OpenLayers keeps a record of all extents loaded (loadedExtentsRtree_), and inserts this skipped extent. So next time the user zooms below the threshold, it does not load anything, because it remembers that this extent was already loaded - in fact it was not.
  There is a removeLoadedExtent method - but at the time of skipping this extent is not there yet.
  I checked the source code, and OpenLayers calls my loader first, and then inserts the extent.</p>
  <p>Does anyone have any idea? I am struggling with this for a while now. Thank you!</p>