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.

test/browser/spec/ol/render/canvas/textbuilder.test.js

@@ -320,7 +320,7 @@ describe('ol.render.canvas.TextBuilder', function () {
     builder.drawText(feature.getGeometry(), feature);
     expect(builder.coordinates).to.have.length(2);
     expect(builder.instructions).to.have.length(3);
-    const geometryWidths = builder.instructions[1][24];
+    const geometryWidths = builder.instructions[1][25];
     expect(geometryWidths).to.have.length(1);
     expect(geometryWidths[0]).to.be(120);
   });