Allow styles to override feature geometries

With this change, application developers are able to define styles that
render a different geometry than the feature geometry. This can e.g. be
used to render an interior point of a polygon instead of the polygon, or
to render symbols like arrows along lines.

externs/olx.js

@@ -6075,7 +6075,8 @@ olx.style.TextOptions.prototype.stroke;
 
 
 /**
- * @typedef {{fill: (ol.style.Fill|undefined),
+ * @typedef {{geometry: (undefined|string|ol.geom.Geometry|function(ol.Feature): ol.geom.Geometry),
+ *     fill: (ol.style.Fill|undefined),
  *     image: (ol.style.Image|undefined),
  *     stroke: (ol.style.Stroke|undefined),
  *     text: (ol.style.Text|undefined),
@@ -6085,6 +6086,15 @@ olx.style.TextOptions.prototype.stroke;
 olx.style.StyleOptions;
 
 
+/**
+ * Feature property or geometry or function returning a geometry to render
+ * for this style.
+ * @type {undefined|string|ol.geom.Geometry|function(ol.Feature): ol.geom.Geometry}
+ * @api
+ */
+olx.style.StyleOptions.prototype.geometry;
+
+
 /**
  * Fill style.
  * @type {ol.style.Fill|undefined}