belt: correct the designed-view preview's belt-Z origin and reject mis-mapped outliers

The Cartesian designed-view preview over-extended the toolpaths past the model
shell by a height-proportional amount (up to ~20mm tall parts), most visibly on
long multi-part prints; compact parts like a calibration cube looked fine.

Two coupled causes:
- Belt start G-code that primes with a Z advance and a 'G92 Z0' reset leaves a
  constant machine-Z origin in the GCodeProcessor, so move positions are stored as
  gcode_Z + origin. The linear back-transform mixes that constant with the
  gantry-Y term, leaving a per-move designed-Y error that min-corner anchoring
  cannot cancel when an elevated move (e.g. a bridge) happens to cancel it at the
  bbox minimum. Expose GCodeProcessorResult::belt_z_origin (the m_origin[Z] left by
  the start G-code) and subtract it before the back-transform.
- Elevated features (bridges/overhangs) are mis-mapped by the linear inverse to
  outside the model body; build the anchor bbox only from moves within model_bb +/-
  10mm, with a fallback to the full bbox when the clip would drop the bulk (object
  placed away from the belt entry) so the gross-offset case still anchors.

Preview-only; G-code output is unchanged.

src/libslic3r/GCode/GCodeProcessor.hpp

@@ -248,6 +248,11 @@ class Print;
         // Belt printer: physical tilt magnitude (deg) parsed from the slicing-rotation
         // header comment; used to enable the preview's belt view.
         float belt_tilt_angle{ 0.f };
+        // Belt printer: machine-Z origin offset (mm) left in m_origin[Z] by the start
+        // G-code (purge-blob belt advance + G92 Z0 resets). Move positions are stored
+        // as gcode_Z + this offset, so the designed-view back-transform must subtract it
+        // to recover the model's belt coordinate.
+        float belt_z_origin{ 0.f };
         RemapAxis preslice_remap_x{ RemapAxis::PosX };
         RemapAxis preslice_remap_y{ RemapAxis::PosY };
         RemapAxis preslice_remap_z{ RemapAxis::PosZ };
@@ -317,6 +322,7 @@ class Print;
             filament_change_count_map = other.filament_change_count_map;
             initial_layer_time = other.initial_layer_time;
             belt_tilt_angle = other.belt_tilt_angle;
+            belt_z_origin = other.belt_z_origin;
             preslice_remap_x = other.preslice_remap_x;
             preslice_remap_y = other.preslice_remap_y;
             preslice_remap_z = other.preslice_remap_z;