Belt printer: address the review on #14394

Code review items (raistlin7447):

1. PrintObject::slice() zeroes m_belt_min_z, m_belt_global_z_offset and
   m_belt_global_xy_correction before slicing. They were only written in belt
   mode, so a project switched to a normal printer, or whose tilt axis was set
   to None, kept the old offsets and shifted the adaptive infill octree and the
   organic support layers by them.
2. TreeModelVolumes shifts the support blockers into the raft-offset index
   space; a test now pins the index the blocker lands on.
3. The final-alignment clamp in libnest2d is opt-in (NfpPConfig::clamp_to_bin)
   and arrange sets it for belt printers only. Printers with an off-centre
   best_object_pos keep their alignment; a flat-bed test pins that.
4. The preview's belt view follows the loaded G-code, not the selected printer:
   GCodeProcessor carries the file's belt keys (and, for a belt file, its bed)
   into export_config_for_render(), and GCodeViewer enables the belt view from
   the header tilt.
5. belt_shift_layer_grid() also shifts the cached belt floor and the global Z
   offset, so a support-only or brim-only change after the purge-prism snap
   matches a fresh slice.
6. update_print_fff_config() resets raft_layers and draft_shield on a belt
   printer instead of only greying out the fields Print::validate() rejects.
7. GCodeWriter takes a first-layer point test instead of the FirstLayerPlane;
   GCode installs one that measures from the belt surface, like its
   extrusions, so the first-layer travel speed and the second-layer
   temperature change no longer depend on the gcode_remap_* convention.
8. belt_brim_clip_leading_edge() is exported and called by both the generator
   and the test.
9. Both phong.vs shaders use slope.up_direction for the overhang highlight.

The pre-slice and G-code axis remaps are gated on belt_printer through
BeltTransformPipeline::axis_remap_enabled(), so belt keys left in a profile
cannot change a non-belt print.

Tests requested in the review: belt-only keys at non-default values leave
non-belt G-code unchanged; switching a sliced project from belt to non-belt
(and tilt axis None) matches a fresh slice; a support-only change on a belt
purge print matches a fresh slice; non-belt start G-code moves keep the
first-layer Z in the processor; the belt brim's segment count catches a band
emitted twice.

The belt-to-non-belt test exposed an unrelated gap: invalidate_step(posSlice)
re-invalidated posSupportMaterial but not posSimplifySupportPath, so after
any re-slice the regenerated support paths were exported unsimplified.
posSimplifySupportPath is now in that list.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/libslic3r/PrintObject.cpp

@@ -1777,7 +1777,10 @@ bool PrintObject::invalidate_step(PrintObjectStep step)
         invalidated |= this->invalidate_steps({ posIroning, posContouring, posSimplifyInfill });
         invalidated |= m_print->invalidate_steps({ psSkirtBrim });
     } else if (step == posSlice) {
-		invalidated |= this->invalidate_steps({ posPerimeters, posPrepareInfill, posInfill, posIroning, posContouring, posSupportMaterial, posSimplifyPath, posSimplifyInfill });
+        // posSimplifySupportPath is listed with posSupportMaterial: invalidate_steps() does not
+        // propagate, so without it a re-slice regenerated the supports but kept the step done,
+        // and the new support paths were exported unsimplified, unlike a fresh slice.
+		invalidated |= this->invalidate_steps({ posPerimeters, posPrepareInfill, posInfill, posIroning, posContouring, posSupportMaterial, posSimplifyPath, posSimplifyInfill, posSimplifySupportPath });
         invalidated |= m_print->invalidate_steps({ psSkirtBrim });
         m_slicing_params.valid = false;
         // The exact belt_floor_z_shift is recomputed when slice() runs again.