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.
These could be calculated from the fill areas if needed.
On the other side, the non-classified (non-split) fill areas are stored
now for use in the "ensure vertical wall thickness" feature,
also the non-split fill areas are re-used when recalculating the infills.
This is safer than trying to stitch the fill region together from the
classified fragments.
Modified the "ensure vertical wall thickness" feature to use the non-split
fill areas instead of perimeter areas for the calculation
of non-supported regions. This is cheaper as the fill areas contain
roughly half the edges.
