open_edges: consider when initializing checking against bounding box first

[TaipanRex]

When open_edges is initialized, the scanline checks intersection against all Edges in graph. It might be faster on graphs with many obstacle polygons to check if the scanline crosses the bounding box of each polygon first. For the boxes it does not cross, all edges in that polygon can be ignored.

[TaipanRex]

Using line_profiler to profile visible_vertices(), about 10% of running time is spent on initializing open_edges, so any improvements won't have a dramatic effect, but still worth exploring. (interesting note: 58% of time is spent on insort and bisect).