Support declutter mode for image styles

Allows to specify for each image style, whether the image
should be decluttered, always drawn but still serving as
obstacle, or drawn without being an obstacle for other
images/texts.

The layer must still have declutter = true set for this
property to have any effect.

src/ol/style/RegularShape.js

@@ -31,6 +31,7 @@ import {
  * @property {boolean} [rotateWithView=false] Whether to rotate the shape with the view.
  * @property {number|import("../size.js").Size} [scale=1] Scale. Unless two dimensional scaling is required a better
  * result may be obtained with appropriate settings for `radius`, `radius1` and `radius2`.
+ * @property {"declutter"|"obstacle"|"none"|undefined} [declutterMode] Declutter mode
  */
 
 /**
@@ -69,6 +70,7 @@ class RegularShape extends ImageStyle {
       scale: options.scale !== undefined ? options.scale : 1,
       displacement:
         options.displacement !== undefined ? options.displacement : [0, 0],
+      declutterMode: options.declutterMode,
     });
 
     /**
@@ -159,6 +161,7 @@ class RegularShape extends ImageStyle {
       rotateWithView: this.getRotateWithView(),
       scale: Array.isArray(scale) ? scale.slice() : scale,
       displacement: this.getDisplacement().slice(),
+      declutterMode: this.getDeclutterMode(),
     });
     style.setOpacity(this.getOpacity());
     return style;