Ability to add custom maptypes with the googlemaps service

Import from https://bugzilla.wikimedia.org/show_bug.cgi?id=44869

It would be great to be able to add custom map types to the googlemaps service. In my specific case, for instance, I would like to overlay the map offered by the Norwegian Mapping Authority (Statens Kartverk) alongside Google's terrain map. Documentation for how to do this can be found here: https://developers.google.com/maps/documentation/javascript/maptypes#CustomMapTypes

Using a practical example, with the Norwegian Mapping authority as described, the Javascript code would/could like like this:

function StatkartMapType(name, layer) {
  this.layer = layer
  this.name = name
  this.alt = name
  this.tileSize = new google.maps.Size(256,256);
  this.maxZoom = 19;
  this.getTile = function(coord, zoom, ownerDocument) {
    var div = ownerDocument.createElement('DIV');
    div.style.width = this.tileSize.width + 'px';
    div.style.height = this.tileSize.height + 'px';
    div.style.backgroundImage = "url(http://opencache.statkart.no/gatekeeper/gk/gk.open_gmaps?layers=" + this.layer + "&zoom=" + zoom + "&x=" + coord.x + "&y=" + coord.y + ")";
    return div;
  };
}

var map;

function initialize() {
  var mapOptions = {
    zoom: 8,
    center: new google.maps.LatLng(60,9),
    mapTypeControlOptions: {
      mapTypeIds: ['topo2', google.maps.MapTypeId.TERRAIN]
    }
  };
  map = new google.maps.Map(document.getElementById("map_canvas"), mapOptions);
  map.mapTypes.set('topo2',new StatkartMapType("S.Kartverk", "topo2"));
  map.setMapTypeId('topo2');
}

-- kvolden@gmail.com 2013-02-11 11:34:08 UTC

Start with the googlemaps service and the Google Maps JavaScript API custom map types documentation linked in the issue. Compare the requested Norwegian Mapping Authority example with the service's current map-type handling; done means users can register and use a custom map type alongside Google's terrain map.