Add renderMode property to Heatmap layer

src/ol/layer/Heatmap.js

@@ -33,6 +33,11 @@ import Style from '../style/Style.js';
  * @property {string|function(module:ol/Feature):number} [weight='weight'] The feature
  * attribute to use for the weight or a function that returns a weight from a feature. Weight values
  * should range from 0 to 1 (and values outside will be clamped to that range).
+ * @property {module:ol/layer/VectorRenderType|string} [renderMode='vector'] Render mode for vector layers:
+ *  * `'image'`: Vector layers are rendered as images. Great performance, but point symbols and
+ *    texts are always rotated with the view and pixels are scaled during zoom animations.
+ *  * `'vector'`: Vector layers are rendered as vectors. Most accurate rendering even during
+ *    animations, but slower performance.
  * @property {module:ol/source/Vector} [source] Source.
  */