isize should be used for Pos

[mpatraw]

Using signedness as a means of range constraint is not ideal, and especially in the case of graphic coordinates where you may want to have something positioned off the screen to the left or top.

[cpjreynolds]

I'm currently in the middle of a fairly major rewrite of the library so we can have a fairly stable set of core functionality and I'll definitely be taking this into consideration, thank you!