Fixed issue with block IDs being returned negative in snapshots.

src/main/java/com/sk89q/worldedit/data/AnvilChunk.java

@@ -124,7 +124,7 @@ public class AnvilChunk implements Chunk {
                 addId = (blocksAdd[section][index >> 2] & 0xF) << 8;
             }
             
-            return blocks[section][index] + addId;
+            return blocks[section][index] & 0xFF + addId;
         } catch (IndexOutOfBoundsException e) {
             throw new DataException("Chunk does not contain position " + pos);
         }