add options.errorUrl in TileLayer, fix #1263

Author: fuzhenn

src/layer/tile/TileLayer.js

@@ -64,6 +64,7 @@ class TileHashset {
  * @property {Boolean}             [options.cascadeTiles=true]      - draw cascaded tiles of different zooms to reduce tiles
  * @property {Number}              [options.zoomOffset=0]           - offset from map's zoom to tile's zoom
  * @property {Number}              [options.tileRetryCount=0]       - retry count of tiles
+ * @property {String}              [options.errorUrl=null]       - image to replace when encountering error on loading tile image
  * @memberOf TileLayer
  * @instance
  */
@@ -72,6 +73,8 @@ const options = {
     'urlTemplate': null,
     'subdomains': null,
 
+    'errorUrl': null,
+
     'repeatWorld': true,
 
     'background' : true,

src/renderer/layer/tilelayer/TileLayerCanvasRenderer.js

@@ -443,6 +443,10 @@ class TileLayerCanvasRenderer extends CanvasRenderer {
             return;
         }
         if (tileImage instanceof Image) {
+            if (this.layer.options['errorUrl']) {
+                tileImage.src = this.layer.options['errorUrl'];
+                return;
+            }
             this.abortTileLoading(tileImage, tileInfo);
         }
         tileImage.loadTime = 0;