Track top left coordinate in WSI for more stable annotation saving

[PLangsteiner]

This might help with saving the annotations if done correctly.
The anchor point is very important for the multi-threading that I implemented.
The anchor point is calculated in such a way, that the multi-threading has a reference access point to the WSI slide.
However, this behavior might not behave as one might expect and if it is read out directly, it can make massive jumps that one would have to adjust for.

Therefore, I would suggest that you track the top-left corner of the viewport based on movement and zooming, instead of directly reading out the anchor point. This would lead to more stable saving of the annotations and you would not need to change the algorithm.