Some further improvements to BlockGeneric

- Only have computers implement custom block drop logic: everything
   else only drops in creative mode.
 - Fix redstone inputs not being received correctly. Introduced in
   8b86a954ee, yes I'm a silly billy.
 - Only update the neighbour which changed.

src/main/java/dan200/computercraft/shared/util/RedstoneUtil.java

@@ -18,7 +18,7 @@ public class RedstoneUtil
     @Deprecated
     public static int getRedstoneOutput( World world, BlockPos pos, EnumFacing side )
     {
-        return world.getRedstonePower( pos, side );
+        return world.getRedstonePower( pos, side.getOpposite() );
     }
 
     @Deprecated