Make local cache KeyBimaps more strict

Before, mapping any global Key to local Long index would just return the
Key's index, disregarding whether the Key actually belonged to the local
cache. Now, it is checked and if the Key is from a different local cache,
null will be returned.

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

@@ -218,9 +218,11 @@ public class VolatileGlobalCellCache implements CacheControl
 			}
 		};
 
-		final KeyBimap< Long, Key > bimap = KeyBimap.< Long, Key >build(
+		final KeyBimap< Long, Key > bimap = KeyBimap.build(
 				index -> new Key( timepoint, setup, level, index ),
-				key -> key.index );
+				key -> ( key.timepoint == timepoint && key.setup == setup && key.level == level )
+						? key.index
+						: null );
 
 		final Cache< Long, Cell< ? > > cache = backingCache
 				.mapKeys( bimap )