Webgl Points / Add support for feature hit detection
Webgl Points / Add support for feature hit detection

For now only `forEachFeatureAtCoordinate` is implemented.

Each time the viewport is rendered, another similar render pass is
done using the specific hit detection instructions. Feature uid's are
encoded in the r,g,b,a channels and can then be decoded on the fly.

Note: the `readPixels` operation is taking a lot of time,
around 10-20ms each frame.

stweil/openlayers,geekdenz/ol3,stweil/ol3,ahocevar/openlayers,geekdenz/openlayers,geekdenz/ol3,ahocevar/openlayers,bjornharrtell/ol3,geekdenz/ol3,stweil/openlayers,geekdenz/ol3,oterral/ol3,tschaub/ol3,tschaub/ol3,openlayers/openlayers,tschaub/ol3,openlayers/openlayers,ahocevar/ol3,ahocevar/ol3,oterral/ol3,ahocevar/ol3,adube/ol3,tschaub/ol3,adube/ol3,ahocevar/ol3,bjornharrtell/ol3,stweil/ol3,stweil/ol3,stweil/openlayers,geekdenz/openlayers,ahocevar/openlayers,openlayers/openlayers,oterral/ol3,stweil/ol3,geekdenz/openlayers,bjornharrtell/ol3,adube/ol3