1) Removed the already commented-out scaling / unscaling when doing
"safe offsetting"
2) Removed some of the "safe offsetting" at calls where it never was used.
3) Reworked Clipper & ClipperUtils to pass Polygons / ExPolygons / Surfaces
as input parameters without conversion to ClipperLib::Paths. This
should save a lot of memory allocation and copying.
4) Reworked conversions from ClipperLib::Paths & PolyTree to Polygons /
ExPolygons to use the move operator to avoid many unnecessary allocations.
5) Reworked some "union with safe ofsetting" to "offset_ex", which should
be cheaper.
Fixed some issues in internal anchors of the Adaptive Cubic infill.
The ugly and dangerous implicit casting operators in Line, MultiPoint,
Polyline and Polygon were made explicit.
Fixes Connecting / expanding Bottom Layers to Vase Perimeter #253
Fixes Slicing error in vase mode #452
Fixes Slicing Issue (Vase Mode, 0.6mm dmr nozzle) #1887
Fixes Top fill pattern isn't used in spiral vase mode #2533
Fixes Cisar's vase doesn't slice correctly, creates artefacts #3595
When the model is sliced, all the contours are newly oriented
counter-clockwise (even holes), merged and then only the largest area
contour is retained. In perimeter generator, if the largest contour
splits into multiple perimeters, newly only the largest area perimeter
is retained in spiral vase mode. These two changes solve #3595 and similar.
The infill is newly calculated only for the bottom solid layers
if the spiral vase mode is active (removes various unwanted infill
along the vase walls), and the last bottom solid layer is switched
to a top solid pattern (solves #2533).
The thin walls are newly enforced to be disabled in spiral vase mode,
and the "ensure vertical shell wall" is enforced in spiral vase mode
to extend the bottom of the vase to the vase hull (fixes#253).
