An object in 9-slice art that crosses a guide and draws symbol art is
placed by the map of the cell its center is in, so it can draw outside
the instance's transformed art box. Bounds measured without placing the
art (the renderer's culling, and placements past the object limit) used
that box alone, so the displaced art could be culled and not drawn.
The fallback now takes in the art box placed by each of the nine cells'
maps (NineSliceMap::placement_bounds), grown by the art's reach as
before.