Moved some math macros (sqr, lerp, clamp) to libslic3r.h

Added UNUSED macro to libslic3r.h, used it to reduce some compile warnings.

Split the Int128 class from Clipper library to a separate file,
extended Int128 with intrinsic types wherever possible for performance,
added new geometric predicates.

Added a draft of new FillRectilinear3, which should reduce overfill near the perimeters in the future.

xs/src/libslic3r/Point.cpp

@@ -88,12 +88,6 @@ Point::nearest_point_index(const Points &points) const
     return this->nearest_point_index(p);
 }
 
-template<typename T>
-inline T sqr(const T x)
-{
-    return x * x;
-}
-
 int Point::nearest_point_index(const PointConstPtrs &points) const
 {
     int idx = -1;