Started on proper date line wrapping

What we want in the end is vector tiles repeated just as raster
tiles. This change only avoids repeated tiles with the same
content being rendered and stored in the cache.

src/ol/renderer/canvas/canvasvectorlayerrenderer.js

@@ -222,6 +222,9 @@ ol.renderer.canvas.VectorLayer.prototype.renderFrame =
   var sketchCanvasRenderer = new ol.renderer.canvas.Renderer(
       sketchCanvas, sketchTransform);
   var renderedFeatures = {};
+  // TODO: wrap date line properly
+  var numCols = Math.ceil((tileRangeExtent.maxX - tileRangeExtent.minX) /
+      tileSize.width / tileResolution);
   var tile, tileContext, tileCoord, key, tileExtent, tileState, x, y;
   // render features by geometry type
   var filters = this.geometryFilters_,
@@ -229,7 +232,7 @@ ol.renderer.canvas.VectorLayer.prototype.renderFrame =
       i, filter, type, features, symbolizer;
   for (x = tileRange.minX; x <= tileRange.maxX; ++x) {
     for (y = tileRange.minY; y <= tileRange.maxY; ++y) {
-      tileCoord = new ol.TileCoord(z, x, y);
+      tileCoord = new ol.TileCoord(z, goog.math.modulo(x, numCols), y);
       key = tileCoord.toString();
       tile = this.tileCache_[key];
       tileExtent = tileGrid.getTileCoordExtent(tileCoord);