consider a reusable context for iup_optimize_deltas

In cases where a font has a large number of variation locations, iup_optimize_deltas is doing a lot of redundant allocation. In particular we clone the point list for each location when this could be shared, and we also allocate a number of hashsets that could be reused.

This feels low priority/is not backed by any profiling, but there is likely some room for improvement here.