Return rather than fail if resolution is not defined
Merge pull request #2527 from pagameba/cluster-fix

Return rather than fail if resolution is not defined
Change ol.TileCoord to an Array
Merge pull request #2539 from elemoine/tilecoord

Change ol.TileCoord to Array.<number>
Do not hit-detect the same feature multiple times

In other words forEachFeatureAtPixel should not call the user-provided callback more than once for a given feature.
Merge pull request #2543 from elemoine/foreachfeature

Do not hit-detect the same feature multiple times