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.
Use opt_tileCoord in ol.source.WMTS tileUrlFunction
Use tmpExtent in ol.source.WMTS tileUrlFunction
Merge pull request #2543 from elemoine/foreachfeature

Do not hit-detect the same feature multiple times
Merge pull request #2528 from fredj/wmts-source-optim

ol.source.WMTS optimization
Remove unnecessary newlines
Remove unnecessary local variable