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/LineSplit.cpp

@@ -1,8 +1,19 @@
 #include "LineSplit.hpp"
 
 #include "AABBTreeLines.hpp"
+#include "libslic3r/ClipperZUtils.hpp"
+#include "libslic3r/Point.hpp"
+#include "libslic3r/Line.hpp"
+#include "libslic3r/ExPolygon.hpp"
 #include "SVG.hpp"
 #include "Utils.hpp"
+#include <limits>
+#include "libslic3r/libslic3r.h"
+#include <algorithm>
+#include <cstddef>
+#include <cassert>
+#include <vector>
+#include <utility>
 
 //#define DEBUG_SPLIT_LINE