There was a mixup with the previous fix to dzsave overlap handling,
correct it and update the test suite.
In the previous revision, dzsave overlapped tiles by overlap and sized
them by tile_size. In fact, tiles should be sized as (tile_size + overlap
* 2), ie. tile_size refers to the number of unique pixels per tile.
See https://github.com/jcupitt/libvips/issues/357