misc: improve building alignment with the foundation

partial fix of #57... we still have work to do
![openage_2014-11-07_05-21-51_00](https://cloud.githubusercontent.com/assets/726447/4950198/6ca87086-6657-11e4-8397-c1a5b34309d0.png)
Is component-wise addition necessary here? I can't quite remember, but if I did my coord stuff right, `drawpos_tile += this->size / 2` should work as well. Same for `this->end_pos`.