Add memory usage based heurisitic for unloading tiles

Adding a maximumMemoryUsage limiter which behaves similar to the
maximumNumberOfLoadedTiles property. When the cache exceeds usage, all
unneeded tiles are unloaded.

Source/Scene/Cesium3DTileset.js

@@ -264,6 +264,7 @@ define([
 
         this._maximumScreenSpaceError = defaultValue(options.maximumScreenSpaceError, 16);
         this._maximumNumberOfLoadedTiles = defaultValue(options.maximumNumberOfLoadedTiles, 256);
+        this._maximumMemoryUsage = defaultValue(options.maximumMemoryUsage, 256);
         this._styleEngine = new Cesium3DTileStyleEngine();
 
         /**
@@ -469,6 +470,7 @@ define([
          * });
          *
          * @see Cesium3DTileset#maximumNumberOfLoadedTiles
+         * @see Cesium3DTileset#maximumMemoryUsage
          * @see Cesium3DTileset#trimLoadedTiles
          */
         this.tileUnload = new Event();
@@ -848,6 +850,43 @@ define([
             }
         },
 
+        /**
+         * The maximum amount of memory in MB that can be used by the tileset.
+         * Tiles not in view are unloaded to enforce this.
+         * <p>
+         * If decreasing this value results in unloading tiles, the tiles are unloaded the next frame.
+         * </p>
+         * <p>
+         * If more tiles than <code>maximumMemoryUsage</code> are needed
+         * to meet the desired screen-space error, determined by {@link Cesium3DTileset#maximumScreenSpaceError},
+         * for the current view than the number of tiles loaded will exceed
+         * <code>maximumMemoryUsage</code>.  For example, if the maximum is 256 MB, but
+         * 300 MB of tiles are needed to meet the screen-space error, then 300 MB of tiles may be loaded.  When
+         * these tiles go out of view, they will be unloaded.
+         * </p>
+         *
+         * @memberof Cesium3DTileset.prototype
+         *
+         * @type {Number}
+         * @default 256
+         *
+         * @exception {DeveloperError} <code>maximumMemoryUsage</code> must be greater than or equal to zero.
+         */
+        maximumMemoryUsage : {
+            get : function() {
+                return this._maximumMemoryUsage;
+            },
+            set : function(value) {
+                //>>includeStart('debug', pragmas.debug);
+                if (value < 0) {
+                    throw new DeveloperError('maximumMemoryUsage must be greater than or equal to zero');
+                }
+                //>>includeEnd('debug');
+
+                this._maximumMemoryUsage = value;
+            }
+        },
+
         /**
          * The tileset's bounding volume.
          *
@@ -1977,13 +2016,16 @@ define([
         var replacementList = tileset._replacementList;
         var tileUnload = tileset.tileUnload;
 
+        var totalMemoryUsage = stats.textureMemorySizeInBytes + stats.vertexMemorySizeInBytes + stats.batchTableMemorySizeInBytes;
+        var maximumMemoryUsageInBytes = tileset._maximumMemoryUsage * 1024 * 1024;
+
         // Traverse the list only to the sentinel since tiles/nodes to the
         // right of the sentinel were used this frame.
         //
         // The sub-list to the left of the sentinel is ordered from LRU to MRU.
         var sentinel = tileset._replacementSentinel;
         var node = replacementList.head;
-        while ((node !== sentinel) && ((replacementList.length > maximumNumberOfLoadedTiles) || trimTiles)) {
+        while ((node !== sentinel) && ((replacementList.length > maximumNumberOfLoadedTiles) || trimTiles || (totalMemoryUsage > maximumMemoryUsageInBytes))) {
             var tile = node.item;
 
             decrementPointAndFeatureLoadCounts(tileset, tile.content);
@@ -1995,6 +2037,7 @@ define([
             replacementList.remove(currentNode);
 
             --stats.numberContentReady;
+            totalMemoryUsage = tileset._statistics.textureMemorySizeInBytes + tileset._statistics.vertexMemorySizeInBytes + tileset._statistics.batchTableMemorySizeInBytes;
         }
     }