Add "UnloadTilesPlugin" to save GPU memory (#874)

* Add "UnloadTilesPlugin" to save memory

* Remove options

* Add to README

* BatchedTilesPlugin: Add option to not dispose of original tiles

* Add batched mesh logic

* Remove event dependency

* Updates to BatchedMesh unload

* Add setting to docs

* Change setTileVisible call

* fixes

* Add ordered "priority" support for plugins

* Add names, priorities, unload callback for batched plugin

* Handle plugins

* README update

* remove set active plugin call

* remove comments

* add comment

* Fix incorrect reversion

* comment update

example/src/plugins/UnloadTilesPlugin.js

@@ -0,0 +1,95 @@
+import { estimateBytesUsed } from '../../../src/three/utilities.js';
+
+// Plugin that disposes tiles on unload to remove them from the GPU, saving memory
+
+// TODO:
+// - abstract the "tile visible" callback so fade tiles can call it when tiles are _actually_ marked as non-visible
+// - add a memory unload function to the tiles renderer that can be called and reacted to by any plugin including BatchedMesh,
+// though this may prevent different options. Something like a subfunction that "disposeTile" calls without full disposal.
+export class UnloadTilesPlugin {
+
+	constructor() {
+
+		this.name = 'UNLOAD_TILES_PLUGIN';
+
+		this.tiles = null;
+		this.estimatedGpuBytes = 0;
+
+	}
+
+	init( tiles ) {
+
+		this.tiles = tiles;
+
+		this._onVisibilityChangeCallback = ( { scene, visible, tile } ) => {
+
+			if ( scene ) {
+
+				const size = estimateBytesUsed( scene );
+				this.estimatedGpuBytes += visible ? size : - size;
+
+				if ( ! visible ) {
+
+					tiles.invokeOnePlugin( plugin => plugin.unloadTileFromGPU && plugin.unloadTileFromGPU( scene, tile ) );
+
+				}
+
+			}
+
+		};
+
+		tiles.forEachLoadedModel( ( scene, tile ) => {
+
+			const visible = tiles.visibleSet.has( tile );
+			this._onVisibilityChangeCallback( { scene, visible } );
+
+		} );
+
+		tiles.addEventListener( 'tile-visibility-change', this._onVisibilityChangeCallback );
+
+	}
+
+	unloadTileFromGPU( scene, tile ) {
+
+		if ( scene ) {
+
+			scene.traverse( c => {
+
+				if ( c.material ) {
+
+					const material = c.material;
+					material.dispose();
+
+					for ( const key in material ) {
+
+						const value = material[ key ];
+						if ( value && value.isTexture ) {
+
+							value.dispose();
+
+						}
+
+					}
+
+				}
+
+				if ( c.geometry ) {
+
+					c.geometry.dispose();
+
+				}
+
+			} );
+
+		}
+
+	}
+
+	dispose() {
+
+		this.tiles.removeEventListener( 'tile-visibility-change', this._onVisibilityChangeCallback );
+		this.estimatedGpuBytes = 0;
+
+	}
+
+}