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/Circle.js

@@ -16,6 +16,7 @@ import RegularShape from './RegularShape.js';
  * (positive rotation clockwise, meaningful only when used in conjunction with a two dimensional scale).
  * @property {boolean} [rotateWithView=false] Whether to rotate the shape with the view
  * (meaningful only when used in conjunction with a two dimensional scale).
+ * @property {"declutter"|"obstacle"|"none"|undefined} [declutterMode] Declutter mode
  */
 
 /**
@@ -41,6 +42,7 @@ class CircleStyle extends RegularShape {
         options.rotateWithView !== undefined ? options.rotateWithView : false,
       displacement:
         options.displacement !== undefined ? options.displacement : [0, 0],
+      declutterMode: options.declutterMode,
     });
   }
 
@@ -59,6 +61,7 @@ class CircleStyle extends RegularShape {
       rotation: this.getRotation(),
       rotateWithView: this.getRotateWithView(),
       displacement: this.getDisplacement().slice(),
+      declutterMode: this.getDeclutterMode(),
     });
     style.setOpacity(this.getOpacity());
     return style;