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/VariableWidth.cpp

@@ -1,4 +1,16 @@
 #include "VariableWidth.hpp"
+#include "ExtrusionEntity.hpp"
+#include "Polyline.hpp"
+#include "Flow.hpp"
+#include "Line.hpp"
+#include <cassert>
+#include "libslic3r.h"
+#include "Point.hpp"
+#include <cmath>
+#include <vector>
+#include <cstddef>
+#include <utility>
+#include <algorithm>
 
 namespace Slic3r {