Add new geometryFunction option

This allows applications to control the geometry that is created from the
drawing sketch. Will e.g. be useful to create a regular polygon instead of
a circle when in Circle mode.

externs/olx.js

@@ -2359,6 +2359,7 @@ olx.interaction.DragZoomOptions.prototype.style;
  *     type: ol.geom.GeometryType,
  *     minPointsPerRing: (number|undefined),
  *     style: (ol.style.Style|Array.<ol.style.Style>|ol.style.StyleFunction|undefined),
+ *     geometryFunction: (ol.interaction.Draw.GeometryFunctionType|undefined),
  *     geometryName: (string|undefined),
  *     condition: (ol.events.ConditionType|undefined),
  *     freehandCondition: (ol.events.ConditionType|undefined)}}
@@ -2417,6 +2418,14 @@ olx.interaction.DrawOptions.prototype.minPointsPerRing;
 olx.interaction.DrawOptions.prototype.style;
 
 
+/**
+ * Function that is called when a geometry's coordinates are updated.
+ * @type {ol.interaction.Draw.GeometryFunctionType|undefined}
+ * @api
+ */
+olx.interaction.DrawOptions.prototype.geometryFunction;
+
+
 /**
  * Geometry name to use for features created by the draw interaction.
  * @type {string|undefined}