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/Fill/FillRectilinear2.cpp

@@ -33,36 +33,6 @@
 
 namespace Slic3r {
 
-#ifndef clamp
-template<typename T>
-static inline T clamp(T low, T high, T x)
-{
-    return std::max<T>(low, std::min<T>(high, x));
-}
-#endif /* clamp */
-
-#ifndef sqr
-template<typename T>
-static inline T sqr(T x)
-{
-    return x * x;
-}
-#endif /* sqr */
-
-#ifndef mag2
-static inline coordf_t mag2(const Point &p)
-{
-    return sqr(coordf_t(p.x)) + sqr(coordf_t(p.y));
-}
-#endif /* mag2 */
-
-#ifndef mag
-static inline coordf_t mag(const Point &p)
-{
-    return std::sqrt(mag2(p));
-}
-#endif /* mag */
-
 // Having a segment of a closed polygon, calculate its Euclidian length.
 // The segment indices seg1 and seg2 signify an end point of an edge in the forward direction of the loop,
 // therefore the point p1 lies on poly.points[seg1-1], poly.points[seg1] etc.