use a local instead of reusing global AF in gvrender_polyline

use a local instead of reusing global AF in gvrender_polyline

There is a static global, AF, that is reused for a number of gvrender functions,
but none need to retain previous data stored in this array. This hack presumably
was from a time when allocators were much slower. Refactoring this into a local
allocation makes this function thread safe and removes the need to unnecessarily
prolong the lifetime of this allocation, thus decreasing Graphviz memory usage.
This commit introduces a -Wshadow warning about AF, but that will be removed
when we soon remove the static global.