Resolve vertex constraints for collapsing edge eid=[a,b]. Generally we would collapse a to b, and set the new position as 0.5*(v_a+v_b). However if aorb are non-deletable, then we want to keep that vertex. This vertex (a or b) will be returned in collapse_to, which is -1 otherwise. If aandb are non-deletable, then things are complicated (and documented below).