HARP-12700: Disposal of POI resources by reference counting. (#1946)

* HARP-12700: Disposal of POI resources by reference counting.

- Share material and texture across pois with same image even if they have different render order.
- Do reference counting on resources to dispose of them when they're not used any longer.

* HARP-12700: Address review comment and fix unit tests.

@here/harp-datasource-protocol/lib/TechniqueParams.ts

@@ -707,11 +707,13 @@ export interface MarkerTechniqueParams extends BaseTechniqueParams {
      */
     poiNameField?: string;
     /**
-     * Name of [[ImageTexture]] definition to use.
+     * The name of either the {@link ImageTexture} in {@link Theme.imageTextures} or the user image
+     * cached in {@link @here/harp-mapview#userImageCache} to be rendered as marker.
      */
     imageTexture?: DynamicProperty<string>;
     /**
-     * Field name to extract imageTexture content from.
+     * Field name to extract imageTexture content from, if imageTexture refers to an
+     * [[ImageTexture]] definition.
      */
     imageTextureField?: string;
     /**

@here/harp-mapview/lib/MapView.ts

@@ -1885,6 +1885,7 @@ export class MapView extends EventDispatcher {
     }
 
     /**
+     * @internal
      * Get the {@link ImageCache} that belongs to this `MapView`.
      *
      * Images stored in this cache are primarily used for POIs (icons) and they are used with the
@@ -1900,15 +1901,15 @@ export class MapView extends EventDispatcher {
     /**
      * Get the {@link ImageCache} for user images that belongs to this `MapView`.
      *
-     * Images added to this cache can be removed if no longer required. If images with identical
-     * names are stored in imageCache and userImageCache, the userImageCache will take precedence.
+     * Images added to this cache can be removed if no longer required.
      */
     get userImageCache(): MapViewImageCache {
         return this.m_userImageCache;
     }
 
     /**
      * @hidden
+     * @internal
      * Get the {@link PoiManager} that belongs to this `MapView`.
      */
     get poiManager(): PoiManager {

@here/harp-mapview/lib/Tile.ts

@@ -966,6 +966,9 @@ export class Tile implements CachedResource {
         }
         this.textElementsChanged = true;
         this.m_pathBlockingElements.splice(0);
+        this.textElementGroups.forEach((element: TextElement) => {
+            element.dispose();
+        });
         this.textElementGroups.clear();
     }