Make ol.style.Style a @struct

And take into account that `styles` may be a style or an array of style.

src/ol/format/kmlformat.js

@@ -2390,10 +2390,10 @@ ol.format.KML.writePlacemark_ = function(node, feature, objectStack) {
     // FIXME the styles returned by the style function are supposed to be
     // resolution-independent here
     var styles = styleFunction.call(feature, 0);
-    if (styles && styles.length > 0) {
-      var style = styles[0];
+    if (styles) {
+      var style = goog.isArray(styles) ? styles[0] : styles;
       if (this.writeStyles_) {
-        properties['Style'] = styles[0];
+        properties['Style'] = style;
       }
       var textStyle = style.getText();
       if (textStyle) {

src/ol/source/imagevectorsource.js

@@ -264,6 +264,9 @@ ol.source.ImageVector.prototype.renderFeature_ = function(feature, resolution, p
     return false;
   }
   var i, ii, loading = false;
+  if (!goog.isArray(styles)) {
+    styles = [styles];
+  }
   for (i = 0, ii = styles.length; i < ii; ++i) {
     loading = ol.renderer.vector.renderFeature(
         replayGroup, feature, styles[i],

src/ol/style/style.js

@@ -19,6 +19,7 @@ goog.require('ol.style.Stroke');
  * feature or layer that uses the style is re-rendered.
  *
  * @constructor
+ * @struct
  * @param {olx.style.StyleOptions=} opt_options Style options.
  * @api
  */