coord: divide negatives as supposed, make rendering pixel perfect

I SPENT TWO HOURS ON THIS!
I'm really tired so here is an explanation: 
http://www.microhowto.info/howto/round_towards_minus_infinity_when_dividing_integers_in_c_or_c++.html

This caused that the 4 "quadrants" of the rendering were misaligned. Not good for perfectionists (?)

This bug went unnoticed until put a checkerboard texture. Also when the game starts it's fine, it triggers when you move the camera. 

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demo pictures look at them in 1:1 because pixels and look in the center, vertical horizontal lines

before:
![openage_2014-11-07_04-19-02_00](https://cloud.githubusercontent.com/assets/726447/4949673/90f9024c-664e-11e4-85e0-25cbbf7fa8c2.png)

after:
![openage_2014-11-07_04-18-27_00](https://cloud.githubusercontent.com/assets/726447/4949675/961656b2-664e-11e4-952b-b08e4afedf47.png)