Merge upstream/hanif/belt-printer-fixes into belt/final-round

Brings in upstream/belt-printer (the Sept 14 main merge) plus Hanif Koh's
21 review-fix commits from PR #15685, on top of the MachineKinematics
refactor and the purge-prism / tree-support / first-layer-speed fixes.

Conflict resolution:
- BeltGCodeWriter is gone (kinematics refactor), so Hanif's plate-offset
  fix for it is ported into GCodeWriter: the first-layer-plane checks in
  travel_to_xy / travel_to_xyz / _travel_to_z now evaluate the plate-local
  point, and BeltGCode::init_belt_writer hands the stored plate origin to
  the writer it installs.
- init_belt_writer(Print&) takes Hanif's signature; the BBL flag is set on
  the surviving writer by GCode::_do_export.
- The shared emit_belt_brim_bands() loop keeps the BeltFloorObjectGuard the
  local branch added, so apron bands classify first-layer height against
  their own object.
- eager_lift keeps effective_type: it now carries set_force_normal_lift().
- GCodeWriter's initializer list follows Hanif's member order with
  m_kinematics in its declared position.
- TreeSupport::detect_overhangs uses Hanif's clamped build_plate_tilt_slope()
  for the non-belt path and the belt shear for the belt path.

src/libslic3r/GCodeWriter.hpp

@@ -22,18 +22,19 @@ public:
     bool multiple_extruders;
 
     GCodeWriter() :
-        multiple_extruders(false), m_curr_filament_extruder(MAXIMUM_EXTRUDER_NUMBER, nullptr),
-        m_curr_extruder_id (-1),
-        m_cached_extruder_idx(0),
-        m_single_extruder_multi_material(false),
-        m_last_acceleration(0), m_max_acceleration(0),m_last_travel_acceleration(0), m_max_travel_acceleration(0),
-        m_last_jerk(0), m_max_jerk_x(0), m_max_jerk_y(0),
-        m_last_bed_temperature(0), m_last_bed_temperature_reached(true),
+        multiple_extruders(false),
         m_lifted(0),
         m_to_lift(0),
         m_to_lift_type(LiftType::NormalLift),
-        m_current_speed(3600), m_is_first_layer(true),
-        m_kinematics(std::make_unique<CartesianKinematics>())
+        m_is_first_layer(true), m_current_speed(3600),
+        m_kinematics(std::make_unique<CartesianKinematics>()),
+        m_cached_extruder_idx(0),
+        m_curr_filament_extruder(MAXIMUM_EXTRUDER_NUMBER, nullptr),
+        m_curr_extruder_id (-1),
+        m_single_extruder_multi_material(false),
+        m_last_acceleration(0), m_max_acceleration(0),m_last_travel_acceleration(0), m_max_travel_acceleration(0),
+        m_last_jerk(0), m_max_jerk_x(0), m_max_jerk_y(0),
+        m_last_bed_temperature(0), m_last_bed_temperature_reached(true)
         {}
     Extruder* filament(size_t extruder_id) { assert(extruder_id < m_curr_filament_extruder.size()); return m_curr_filament_extruder[extruder_id]; }
     const Extruder* filament(size_t extruder_id) const { assert(extruder_id < m_curr_filament_extruder.size()); return m_curr_filament_extruder[extruder_id]; }
@@ -255,7 +256,6 @@ private:
 
 
     //BBS
-    unsigned int    m_last_additional_fan_speed;
     int             m_last_bed_temperature;
     bool            m_last_bed_temperature_reached;