title: Tiling an image up into XYZ tiles
body: <p>I cant find any specifications for the XYZ tiles source used by OpenLayers</p>
  
  <p>I have some images where I for each know the 4 corners in the a Mercator projection (EPSG:3857) </p>
  
  <p>I would like to tile those images into XYZ directories, but are not sure how I find the corners of each xyz tile at each zoom lvl. If I can calculate all the xyz tiles corners also in EPSG:3857 coordinates, I simply could cut out the intersection parts with the 4 corners of the images.</p>