Adjustable tilelayer horizontal and vertical pixel offset

This is an initial implementation of the feature described on the title. The only renderer that currently renders the map with the offseted layers is the isometric one.
The intention was to cover the feature described in #4, but adding the flexibility to use an arbitrary offset instead of only a positive vertical one.

Some caveats:

* For **positive vertical** and **negative horizontal** offsets, the current behavior when drawing is to *push* the rest of the tile layers on the opossite direction. Object layers are independent of the offsets since they use pixel positions, which will cause the objects to "fall out of place" on those cases (they are actually staying in place while everything else moves).
* I tried to adpt the bounding and exposed rects to the offsets in a given map and the current layer, but I'm not completely sure I did a good work, so that may hit on performance (maybe?).

There's no hurry, check the changes whenever you've got some time!