Drop a hole that no ring can parent instead of failing the run (#401)

* wagyu: drop a hole no remaining ring can parent instead of throwing

correct_tree() throws "Could not properly place hole to a parent" when
topology correction leaves a hole whose parent ring was removed (degenerate
input such as stacked duplicate rings from coalesced tiny-polygon
placeholders). That aborts the entire tiling run over one unrepresentable
sliver. Remove the ring and its points instead, matching how other
unresolvable degeneracies are handled.

* Add a regression test for dropping an unplaceable hole

A fuzzer-minimized pair of mutually reversed self-intersecting rings that
makes wagyu's correct_tree fail to find a parent for a hole — the same
failure reported in mapbox/tippecanoe#761. Before the topology_correction
change, running this test exits with EXIT_IMPOSSIBLE via the polygon
cleaning error handler; with it, the clean returns.

mapbox/geometry/wagyu/topology_correction.hpp

@@ -1296,7 +1296,10 @@ void correct_tree(ring_manager<T>& manager) {
         }
         if (!found) {
             if ((*itr)->is_hole()) {
-                throw std::runtime_error("Could not properly place hole to a parent.");
+                // No remaining ring contains this hole (degenerate input, e.g. stacked
+                // duplicate rings from coalesced tiny-polygon placeholders); drop it
+                // rather than fail the entire tiling run.
+                remove_ring_and_points(*itr, manager, false);
             } else {
                 // Assign to base of tree by passing nullptr
                 reassign_as_child(*itr, static_cast<ring_ptr<T>>(nullptr), manager);