Fix anchor origin calculation

This PR is an alternative to this fix: https://github.com/openlayers/ol3/pull/1802 , because it breaks some others KML files.

According to the KML spec the origin of anchor is bottom-left, the bug mentioned [in the mailing list ](https://groups.google.com/d/msg/ol3-dev/jr6f2s0k1wU/jRzhgBBWca0J)  was caused by a miscalculation of the y coordinate of the anchor.

So this PR revert the fix and fix the calculation.

I've tested with these files:

[FullCourse.kml ](https://docs.google.com/file/d/0B4uRuJ1hajZmbVA4dWUzUmhfR1VTRlMwdVV5YjZxakRoMEJn/edit)

[Swissmetnet.kml ] (https://docs.google.com/file/d/0B4uRuJ1hajZmS3RTWUM4SmthMzg/edit)

Hope this makes sense.