add method to get LoadingVolatileCache that backs VolatileGlobalCellCache.

This is for bigdataviewer-servert o directly issue Cell requests without
requiring CellImg and associated VolatileCellCache.

src/main/java/bdv/img/cache/VolatileGlobalCellCache.java

@@ -38,6 +38,7 @@ import bdv.cache.WeakSoftCache;
 import bdv.cache.util.BlockingFetchQueues;
 import bdv.img.cache.VolatileImgCells.CellCache;
 import net.imglib2.img.basictypeaccess.volatiles.VolatileAccess;
+import net.imglib2.img.cell.CellImg;
 
 public class VolatileGlobalCellCache implements CacheControl
 {
@@ -176,6 +177,20 @@ public class VolatileGlobalCellCache implements CacheControl
 		volatileCache.invalidateAll();
 	}
 
+	/**
+	 * <em>For internal use.</em>
+	 * <p>
+	 * Get the {@link LoadingVolatileCache} that handles cell loading. This is
+	 * used by bigdataviewer-server to directly issue Cell requests without
+	 * having {@link CellImg}s and associated {@link VolatileCellCache}s.
+	 *
+	 * @return the cache that handles cell loading
+	 */
+	public LoadingVolatileCache< Key, VolatileCell< ? > > getLoadingVolatileCache()
+	{
+		return volatileCache;
+	}
+
 	/**
 	 * A {@link VolatileCacheValueLoader} for one specific {@link VolatileCell}.
 	 */