(non-spiral vase layers) were being closed.
Fixes Bad slicing in vase mode (unexpected bridge and solid infill layers) #3326
Fixes Model with holes in the base does not slice properly in "Vase Mode" #5359
Layer newly remembers bounding boxes of slices,
the bounding boxes are used by G-code generator & newly the support
generator.
Slices are stored as ExPolygons, not ExPolygonCollection.