If the Elephant foot compensation is applied to the 1st object's layer,
the uncompensated 1st object's slice is newly used for calculation
of bridges, overhans, skirt, brim, raft and supports.
Layer::slices were renamed to Layer::lslices to simplify reading
of the code, to differentiate from LayerRegion::slices.
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.
Fix of "Bridging infill not connecting with infill set to 0%" #1301
Top, bottom and bridging areas are extended into infill as long
as the infill is not zero. If the infill is zero,
top, bottom and bridging areas wound not expand into these "void" areas.
With this commit, the top, bottom and bridging areas are allowed to expand
into the "void" areas as long as these "void" areas are supported below
with perimeters or some other non-empty infill, and slightly beyond
these supporting areas into the voids (currently hard coded to 1mm).
