FIX: memory leak when calling add_data in loop

Fixes the memory leak from #88 by removing previously added surfaces from the scene. For some reason the memory usage still slightly increases but much less, i.e., for the following test

https://gist.github.com/mluessi/8658369

the memory goes up to 2.7GB with `num=50, remove=False` and to  600MB with `num=50, remove=True`. But when using  `num=10, remove=True` it needs only 450MB.