Make tile pre-loading a per-layer option

src/ol/layer/tilelayer.exports

@@ -1 +1 @@
-@exportClass ol.layer.TileLayer ol.layer.LayerOptions
+@exportClass ol.layer.TileLayer ol.layer.TileLayerOptions

src/ol/layer/tilelayer.js

@@ -4,21 +4,58 @@ goog.require('ol.layer.Layer');
 goog.require('ol.source.TileSource');
 
 
+/**
+ * @enum {string}
+ */
+ol.layer.TileLayerProperty = {
+  PRELOAD: 'preload'
+};
+
+
 
 /**
  * @constructor
  * @extends {ol.layer.Layer}
- * @param {ol.layer.LayerOptions} layerOptions Layer options.
+ * @param {ol.layer.TileLayerOptions} options Options.
  */
-ol.layer.TileLayer = function(layerOptions) {
-  goog.base(this, layerOptions);
+ol.layer.TileLayer = function(options) {
+
+  goog.base(this, options);
+
+  this.setPreload(
+      goog.isDef(options.preload) ? options.preload : false);
+
 };
 goog.inherits(ol.layer.TileLayer, ol.layer.Layer);
 
 
+/**
+ * @return {boolean} Preload.
+ */
+ol.layer.TileLayer.prototype.getPreload = function() {
+  return /** @type {boolean} */ (this.get(ol.layer.TileLayerProperty.PRELOAD));
+};
+goog.exportProperty(
+    ol.layer.TileLayer.prototype,
+    'getPreload',
+    ol.layer.TileLayer.prototype.getPreload);
+
+
 /**
  * @return {ol.source.TileSource} Source.
  */
 ol.layer.TileLayer.prototype.getTileSource = function() {
   return /** @type {ol.source.TileSource} */ (this.getSource());
 };
+
+
+/**
+ * @param {boolean} preload Preload.
+ */
+ol.layer.TileLayer.prototype.setPreload = function(preload) {
+  this.set(ol.layer.TileLayerProperty.PRELOAD, preload);
+};
+goog.exportProperty(
+    ol.layer.TileLayer.prototype,
+    'setPreload',
+    ol.layer.TileLayer.prototype.setPreload);