New ClipperUtils functions: opening(), closing() as an alternative

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.

src/libslic3r/Fill/Fill.cpp

@@ -252,11 +252,11 @@ std::vector<SurfaceFill> group_fills(const Layer &layer)
 	        // Corners of infill regions, which would not be filled with an extrusion path with a radius of distance_between_surfaces/2
 	        Polygons collapsed = diff(
 	            surfaces_polygons,
-	            offset2(surfaces_polygons, (float)-distance_between_surfaces/2, (float)+distance_between_surfaces/2 + ClipperSafetyOffset));
+				opening(surfaces_polygons, float(distance_between_surfaces /2), float(distance_between_surfaces / 2 + ClipperSafetyOffset)));
 	        //FIXME why the voids are added to collapsed here? First it is expensive, second the result may lead to some unwanted regions being
 	        // added if two offsetted void regions merge.
 	        // polygons_append(voids, collapsed);
-	        ExPolygons extensions = intersection_ex(offset(collapsed, (float)distance_between_surfaces), voids, ApplySafetyOffset::Yes);
+	        ExPolygons extensions = intersection_ex(expand(collapsed, float(distance_between_surfaces)), voids, ApplySafetyOffset::Yes);
 	        // Now find an internal infill SurfaceFill to add these extrusions to.
 	        SurfaceFill *internal_solid_fill = nullptr;
 			unsigned int region_id = 0;