Improve render speed for tile level < 17

In tile levels smaller than 17 the vast majority of items has a
default z value of 0.0. Those can be spared from sorting, which
has a noticable performance impact. Sorting took roughly 10%
of runtime before even though barely any change was needed. Now it
does not turn up anymore in performance measurement -- except for
level 17 where buildings use the z value to sort by their height,
and this is indeed needed.