belt: render the G-code preview in model (Cartesian) space

On a belt printer the emitted G-code is in the machine frame (45-deg sheared,
axis-remapped, scaled), so the toolpath preview shows the print as a sheared
slab floating off the bed. Map each toolpath vertex back to model/Cartesian
space for the "designed" view.

The back-transform is the inverse of the full G-code forward pipeline
(BeltGCodeWriter::to_machine_coords):
  model = [BeltForward^-1 if !gcode_back_transform] . AxisRemap^-1 . MachineFrame^-1
built from config, so it handles any rotation / shear / scale / axis-remap
combination, not just plain 45-deg belt slicing. Computed in load_as_gcode()
from print.config() and applied per-vertex inside libvgcode::convert (display
position only; layer_id, times and the volumetric/flow math keep the raw
machine values, so the layer slider and stats are unaffected).

- Toggle with the existing "Show designed view" checkbox / hotkey B; off shows
  the raw machine-frame G-code (useful for debugging the transform itself).
  Defaults to on.
- Belt printers skip the same-result-id load cache so the upright view applies
  and the toggle takes effect even when the G-code is unchanged.
- The object extrusions (layer_id >= 1) are anchored to the belt entry to drop
  the constant machine-origin offset (start-G-code belt advance) that the linear
  back-transform alone does not capture; start-G-code prime lines are excluded
  so they don't steal the anchor.

src/slic3r/GUI/GCodeViewer.hpp

@@ -245,7 +245,8 @@ mutable bool m_no_render_path { false };
 
     bool  m_belt_view_enabled = false;
     float m_belt_angle_deg = 0.f;
-    bool  m_belt_show_designed = false;  // Toggle: show designed (upright) view via inverse shear
+    bool  m_belt_show_designed = true;   // Toggle: designed (upright, back-transformed) view by default;
+                                         // turn off (hotkey B) to inspect the raw machine-frame G-code.
     Transform3d m_belt_inverse_transform{Transform3d::Identity()};
 
     libvgcode::Viewer m_viewer;