Remove Unused Project Includes and Forward-Declare Where a Type Is Only Referenced

Generated with include-what-you-use and applied conservatively. Only OrcaSlicer's own headers, the ones under src/ and tests/, are removed or forward-declared; standard-library and third-party includes are left alone. An include is removed only when both the Release and the Debug configuration leave it unused, never from inside a conditional block, and never from a file with platform-specific blocks, which only gain includes. Files whose only use of a header sits behind a feature or debug macro (libvgcode's OpenGL ES and marker code, the ARACHNE/TESTS_EXPORT_SVGS debug output) keep their includes.

clonable_ptr.hpp gains #pragma once; it had no include guard and was only safe while Config.hpp was its sole includer.

src/libslic3r/Fill/FillGyroid.cpp

@@ -1,7 +1,5 @@
 #include "../ClipperUtils.hpp"
 #include "../MarchingSquares.hpp"
-#include "../ShortestPath.hpp"
-#include "../Surface.hpp"
 #include <cmath>
 #include <algorithm>
 #include <cstddef>
@@ -18,6 +16,7 @@
 #include "libslic3r/libslic3r.h"
 #include "libslic3r/Polyline.hpp"
 #include "FillGyroid.hpp"
+#include "libslic3r/Polygon.hpp"
 
 // ---------------------------------------------------------------------------
 // Marching-squares scalar field for the optimized gyroid branch.