Add Missing Includes Across src/libslic3r

Every libslic3r source and header now directly includes the headers declaring what it uses, rather than relying on the precompiled header or transitive includes. Generated with clang-tidy misc-include-cleaner, with libslic3r headers spelled libslic3r/... so they resolve outside the library's private include paths. MultiMaterialSegmentation.hpp, Support/SupportParameters.hpp and Format/STEP.hpp are made self-contained by hand.

src/libslic3r/Algorithm/RegionExpansion.cpp

@@ -1,11 +1,25 @@
 #include "RegionExpansion.hpp"
+#include "libslic3r/Point.hpp"
+#include "libslic3r/ExPolygon.hpp"
+#include "libslic3r/libslic3r.h"
+#include "libslic3r/Polyline.hpp"
+#include "libslic3r/Polygon.hpp"
 
+#include <algorithm>
+#include <cmath>
+#include <cstddef>
+#include <cassert>
+#include <iterator>
+#include <cstdint>
 #include <libslic3r/AABBTreeIndirect.hpp>
 #include <libslic3r/ClipperZUtils.hpp>
 #include <libslic3r/ClipperUtils.hpp>
 #include <libslic3r/Utils.hpp>
 
+#include <math.h>
 #include <numeric>
+#include <vector>
+#include <utility>
 
 namespace Slic3r {
 namespace Algorithm {