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/ElephantFootCompensation.cpp

@@ -8,7 +8,6 @@
 #include "ElephantFootCompensation.hpp"
 #include "Flow.hpp"
 #include "Geometry.hpp"
-#include "SVG.hpp"
 #include "Utils.hpp"
 
 #include <algorithm>
@@ -19,6 +18,7 @@
 #include <utility>
 #include <math.h>
 #include <limits>
+#include "ExtrusionEntity.hpp"
 
 // #define CONTOUR_DISTANCE_DEBUG_SVG