Merge upstream/main into belt-printer

Brings belt-printer up to main 4b4a261787. Resolutions:

- G-code header (#15897, #15915): main moved the header, config and
  thumbnail block later in _do_export; write_belt_header() moves with it,
  still after the thumbnails and outside the BTT_TFT gate.
- _extrude: first-layer acceleration keeps the per-path first-layer plane
  test with main's cached nozzle index (#16028); main's set_speed out-param
  form (#16108) everywhere else.
- GCodeWriter (#16108): the arc-to-polyline fallback for machine mappings
  that cannot express G2/G3 now runs in the out-param extrude_arc_to_xy,
  which is the overload GCode calls, and appends to the caller's string.
- GCodeProcessorResult: the belt fields join main's forwarding assign.
- Clipper2 (#15969): belt arrange helpers take Slic3r::Point; the tree
  support join types lose their ClipperLib qualifier.
- CLI arrange (#15837): belt printers still reserve no wipe tower.
- Wipe tower options (#15841): the two new sparse-layer toggles are hidden
  for belt printers like the rest of the tower options.
- Keyboard shortcuts (#15706): main's registry replaces the old key switch;
  the belt view toggle is re-registered in the next commit.
- Print::process: the belt purge-plan undo runs before main's SliceStarted
  event.
- scripts/filament_id_snapshot.json: deleted on main (a77209af8f).
- Includes and appended tests: union of both sides.

src/libslic3r/Support/SupportMaterial.cpp

@@ -1,23 +1,49 @@
 #include "ClipperUtils.hpp"
+#include "libslic3r/ExPolygon.hpp"
 #include "ExtrusionEntity.hpp"
 #include "ExtrusionEntityCollection.hpp"
+#include "libslic3r/Flow.hpp"
 #include "Layer.hpp"
+#include "libslic3r/Polygon.hpp"
+#include "libslic3r/Polyline.hpp"
 #include "Print.hpp"
 #include "SupportMaterial.hpp"
+#include "libslic3r/Slicing.hpp"
+#include "libslic3r/Support/SupportLayer.hpp"
+#include "libslic3r/PrintConfig.hpp"
 #include "SupportCommon.hpp"
 #include "BeltFloorContext.hpp"
 #include "Geometry.hpp"
 #include "Point.hpp"
 #include "MutablePolygon.hpp"
+#include "libslic3r/libslic3r.h"
+#include "libslic3r/Surface.hpp"
+#include "libslic3r/TriangleSelector.hpp"
 
+#include <agg/agg_rendering_buffer.h>
+#include <agg/agg_renderer_base.h>
+#include <algorithm>
+#include <cfloat>
 #include <cmath>
+#include <iterator>
+#include <cstdint>
+#include <cstddef>
+#include <deque>
+#include <initializer_list>
+#include <limits>
+#include <map>
 #include <memory>
 #include <boost/log/trivial.hpp>
 #include <boost/container/static_vector.hpp>
 
+#include <set>
+#include <numeric>
 #include <tbb/parallel_for.h>
 #include <tbb/spin_mutex.h>
 #include <tbb/task_group.h>
+#include <vector>
+#include <utility>
+#include <tuple>
 
 #define SUPPORT_USE_AGG_RASTERIZER
 
@@ -64,9 +90,9 @@ namespace Slic3r {
 #define PILLAR_SIZE (2.5)
 #define PILLAR_SPACING 10
 
-//#define SUPPORT_SURFACES_OFFSET_PARAMETERS ClipperLib::jtMiter, 3.
-//#define SUPPORT_SURFACES_OFFSET_PARAMETERS ClipperLib::jtMiter, 1.5
-#define SUPPORT_SURFACES_OFFSET_PARAMETERS ClipperLib::jtSquare, 0.
+//#define SUPPORT_SURFACES_OFFSET_PARAMETERS jtMiter, 3.
+//#define SUPPORT_SURFACES_OFFSET_PARAMETERS jtMiter, 1.5
+#define SUPPORT_SURFACES_OFFSET_PARAMETERS jtSquare, 0.
 
 static constexpr bool support_with_sheath = false;
 
@@ -1691,7 +1717,7 @@ static inline std::tuple<Polygons, Polygons, double> detect_contacts(
                         offset(
                             diff_polygons,
                             scaled<float>(SUPPORT_MATERIAL_MARGIN / NUM_MARGIN_STEPS),
-                            ClipperLib::jtRound,
+                            jtRound,
                             // round mitter limit
                             scale_(0.05)),
                         slices_margin.polygons);