Support BBOX strategy in the GeoJSON layer type

Started at #1370448: Provide GeoJSON Layers from Views (integrating with views_geojson)

@jeffschuler's code:

This diff is against OL + patch in #16:

diff --git a/plugins/layer_types/openlayers_layer_type_geojson.js b/plugins/layer_types/openlayers_layer_type_geojson.js
index a2eb38e..f9750cf 100644
--- a/plugins/layer_types/openlayers_layer_type_geojson.js
+++ b/plugins/layer_types/openlayers_layer_type_geojson.js
@@ -12,7 +12,6 @@ Drupal.openlayers.layer.geojson = function(title, map, options) {
   var features = null;
   options.projection = 'EPSG:' + options.projection;
   options.styleMap = Drupal.openlayers.getStyleMap(map, options.drupalID);
-  var layer = new OpenLayers.Layer.Vector(title, options);

   // GeoJSON Projection handling
   var geojson_options = {
@@ -20,6 +19,15 @@ Drupal.openlayers.layer.geojson = function(title, map, options) {
     'externalProjection': new OpenLayers.Projection(options.projection)
   };

+  // Use BBOX strategy.
+  options.strategies = [ new OpenLayers.Strategy.BBOX() ];
+  options.protocol = new OpenLayers.Protocol.HTTP({
+    url: options.url,
+    format: new OpenLayers.Format.GeoJSON(geojson_options),
+  });
+
+  var layer = new OpenLayers.Layer.Vector(title, options);
+
   // If GeoJSON data is provided with the layer, use that
   if (options.geojson_data) {
     // Read data in.