Fix tilesize calculation

[thesocialdev]

The way it is calculated makes abaculus work only for scale = 1.

Also, changing the test case with an example easier to check and to reflect the new calculations.

The inputs are the same as the following: https://maps.wikimedia.org/img/osm-intl,5,-24.405,-47.368,250x250.png

The tile outputs are the following: https://maps.wikimedia.org/osm-intl/5/11/17.png https://maps.wikimedia.org/osm-intl/5/12/17.png https://maps.wikimedia.org/osm-intl/5/11/18.png https://maps.wikimedia.org/osm-intl/5/12/18.png