Fixes #17 - Marker's anchors are now integers

[ffranchina]

Hi Andrea :)

I ran into the issue described in #17 and I managed to solve it.

From the specs of pyglet's ImageData, I noticed that both anchor_x and anchor_y are expected to be int. Therefore I forced an integer division by using the floor division operator to ensure that the result matches the expected type.

I hope it can help! Thanks for the library :)