Additional index dimension for RTree; use RTree again

The RTree can easily maintain an additional index dimension,
by passing a type with each added item. Now instead of
maintaining an RTree for each geometry type, we have a single
RTree with a type filter. With this change, using the RTree
finally speeds up rendering as expected.

src/ol/renderer/canvas/canvasvectorlayerrenderer.js

@@ -327,7 +327,8 @@ ol.renderer.canvas.VectorLayer.prototype.renderFrame =
         for (i = 0; i < numFilters; ++i) {
           geomFilter = filters[i];
           type = geomFilter.getType();
-          features = layer.getFeatures(geomFilter);
+          features = layer.getFeatures(new ol.filter.Logical(
+              [geomFilter, extentFilter], ol.filter.LogicalOperator.AND));
           if (features.length) {
             groups = layer.groupFeaturesBySymbolizerLiteral(features);
             numGroups = groups.length;