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/ShortEdgeCollapse.cpp

@@ -1,10 +1,14 @@
 #include "ShortEdgeCollapse.hpp"
+#include "Point.hpp"
+#include "TriangleMesh.hpp"
 #include "libslic3r/NormalUtils.hpp"
 
+#include <cstddef>
 #include <unordered_map>
 #include <unordered_set>
 #include <random>
 #include <algorithm>
+#include <vector>
 
 namespace Slic3r {