Part 2.7: Add G-code back-transform and tree support belt floor clipping

- Add BeltBackTransform class that inverts the shear/scale matrix and
  applies it in GCodeWriter::to_machine_coords() so G-code outputs in
  the machine's physical coordinate space, gated by new
  belt_gcode_back_transform config option
- Extend belt floor clipping to all three tree support pipelines
  (Prusa-style, Orca organic, TreeModelVolumes) with per-layer polygon
  clipping, anti-overhang integration, and belt raft extension layers
- Fix tree drop_nodes() belt termination, organic support global Z
  offset, collision calculation index bug, and first-layer brim/empty
  layer checks for belt printers

two-shot - first build built but didn't plumb to UI.  Woah.

add pre-slice axis remap, because Y needs to be Z

going to change tactic and move based on bbox min

switch to per axis snapping

per axis swap snap now per object

build plate tilt wasn't invalidating slicer settings

support upper bound now correct, need to get lower bound corrected

axis swapped support termination corrected

Z Shear works with and without pre-slice remap now

src/libslic3r/GCode.hpp

@@ -493,6 +493,11 @@ private:
        This affects the input arguments supplied to the extrude*() and travel_to()
        methods. */
     Vec2d                               m_origin;
+    // Per-axis origin snap: shift G-code so each object's bbox min = offset.
+    bool                                m_origin_snap[3] = {false, false, false};
+    double                              m_origin_snap_offset[3] = {0., 0., 0.};
+    // Called when switching instances to recompute the writer's snap for this instance.
+    void update_origin_snap(const PrintObject *obj, const Point &inst_shift);
     FullPrintConfig                     m_config;
     DynamicConfig                       m_calib_config;
     // scaled G-code resolution