Use geometry type enum for configuring draw interaction

This allows us to cast single-part geometries to multi-part types before adding features to the target layer.  This doesn't yet allow for drawing multi-part geometries with multiple parts.  That can be handled separately.

examples/draw-features.html

@@ -35,7 +35,7 @@
           <p id="shortdesc">Example of using the Draw interaction.</p>
           <form class="form-inline">
             <label>Geometry type &nbsp;</label>
-              <select id="mode">
+              <select id="type">
                 <option value="polygon">Polygon</option>
                 <option value="linestring">Linestring</option>
                 <option value="point">Point</option>

examples/draw-features.js

@@ -97,24 +97,24 @@ var map = new ol.Map({
   })
 });
 
-var modeSelect = document.getElementById('mode');
+var typeSelect = document.getElementById('type');
 
 var draw; // global so we can remove it later
 function addInteraction() {
   draw = new ol.interaction.Draw({
     layer: vector,
-    mode: /** @type {ol.interaction.DrawMode} */
-        (modeSelect.options[modeSelect.selectedIndex].value)
+    type: /** @type {ol.geom.GeometryType} */
+        (typeSelect.options[typeSelect.selectedIndex].value)
   });
   map.addInteraction(draw);
 }
 
 
 /**
- * Let user change the draw mode.
+ * Let user change the geometry type.
  * @param {Event} e Change event.
  */
-modeSelect.onchange = function(e) {
+typeSelect.onchange = function(e) {
   map.removeInteraction(draw);
   addInteraction();
 };