Move useLowResolutionTiles from layer renderer to tile source

src/ol/source/tilesource.js

@@ -121,6 +121,25 @@ ol.source.TileSource.prototype.getTileGrid = function() {
 };
 
 
+/**
+ * @param {number} z Z.
+ * @param {ol.Extent} extent Extent.
+ */
+ol.source.TileSource.prototype.useLowResolutionTiles = function(z, extent) {
+  var tileGrid = this.getTileGrid();
+  var tileRange, x, y, zKey;
+  // FIXME this should loop up to tileGrid's minZ when implemented
+  for (; z >= 0; --z) {
+    tileRange = tileGrid.getTileRangeForExtentAndZ(extent, z);
+    for (x = tileRange.minX; x <= tileRange.maxX; ++x) {
+      for (y = tileRange.minY; y <= tileRange.maxY; ++y) {
+        this.useTile(new ol.TileCoord(z, x, y));
+      }
+    }
+  }
+};
+
+
 /**
  * Marks a tile coord as being used, without triggering a load.
  * @param {ol.TileCoord} tileCoord Tile coordinate.