flip direction of the infill layer with each interface layer.
Unfortunately the flipping of support interface directions may not work
reliably due to base support layer heights growing at different rate
from the interface layers.
for offset2() with clear meaning.
New ClipperUtils functions: expand(), shrink() as an alternative
for offset() with clear meaning.
All offset values for the new functions are positive.
Various offsetting ClipperUtils (offset, offset2, offset2_ex) working
over Polygons were marked as unsafe, sometimes producing invalid output
if called for more than one polygon. These functions were reworked
to offset polygons one by one. The new functions working over Polygons
shall work the same way as the old safe ones working over ExPolygons,
but working with Polygons shall be computationally more efficient.
Improvements in FDM support generator:
1) For both grid and snug supports: Don't filter out supports for which
the contacts are completely reduced by support / object XY separation.
2) Rounding / merging of supports using the closing radius parameter is
now smoother, it does not produce sharp corners.
3) Snug supports: When calculating support interfaces, expand the projected
support contact areas to produce wider, printable and more stable interfaces.
4) Don't reduce support interfaces for snug supports for steep overhangs,
that would normally not need them. Snug supports often produce very
narrow support interface regions and turning them off makes the support
interfaces disappear.
Fixed crash with zero support base spacing.
The bug fix was not correct and it disabled the new "zig-zag" sparse
infill generator, leading to GH issue #7014
Somehow improved missing interface layers for snug supports
by propagating full overhangs when generating interface layers.
Fixed generation of soluble interfaces for support enforcers,
where base support was used for steeper overhangs.
Disabled filtering out thin regions from the lower layer, that will
not be covered by perimeters, thus they are not supporting the current layer.
However this may lead to a situation where regions at the current layer
that are narrow thus not extrudable will generate unnecessary supports.
For example, see GH issue #3094
