Re-Add libslic3r Includes After the Clipper2 2.0.1 Migration

Rebasing onto main took main's version of the files the Clipper2 migration rewrote, so their added includes are restored here, along with includes for main's new code. Clipper2's individual headers are now ignored by clang-tidy: they only build the Z variant through clipper2_z.hpp, which defines USINGZ first, so including clipper.core.h and the like directly broke ClipperZUtils.cpp.

src/libslic3r/ClipperZUtils.cpp

@@ -1,6 +1,8 @@
 #include "ClipperZUtils.hpp"
+#include "ClipperUtils.hpp"
 
 #include <clipper2/clipper2_z.hpp>
+#include <cstddef>
 
 namespace Slic3r {
 namespace ClipperZUtils {