Part 2: Replace belt rotation w/ per-axis shear transforms and G-code axis remap

- Replace monolithic belt rotation transform with independent per-axis
    shear controls (mode/angle/source-axis for X, Y, Z) and G-code axis
    remapping, giving full flexibility to match any belt printer's
    coordinate system
  - Remove all rotation mode logic and intermediate type+axes dropdowns,
    simplifying the pipeline to pure shear matrices while preserving the
    default behavior (Y += Z*cot(45deg) with identity remap)
  - Clean up GCodeWriter, GCodeProcessor, and GCodeViewer for the new
    shear-only model; expose 12 new settings in printer UI via
    Tab.cpp/Preset.cpp

Implement belt printer tilted slicing

Implement the core belt slicing pipeline that makes the slicer
tilt-aware:

Step 1: GCodeWriter::to_machine_coords() - R(+alpha, X) rotation
  from slicing frame to machine frame
Step 2: PrintObject - belt-rotated object height calculation
  (y*sin(a) + z*cos(a)) for correct layer count
Step 3: PrintObjectSlice - apply R(-alpha, X) rotation trafo so
  horizontal slice planes correspond to belt-parallel planes,
  with Z-shift computed from model volumes
Step 4: GCodeProcessor - machine-frame preview (no transform needed)
Step 5: 3DBed - rotate bed visualization about X by belt angle

Fix: belt surface IS the build plate, no mesh rotation

Currently still slicing perpendicular to the belt normal.  Need to figure out why.

Fix G-code Z sign: use R(-alpha, X) so Z+ is away from belt

The previous R(+alpha, X) transform produced negative Z values
(-y*sin(a) term dominated). Changed to R(-alpha, X) which gives
machine_z = y*sin(a) + z*cos(a), always positive for points
above the belt surface. Z increases with each layer as expected.

reverting and changing slice methodology

Add pink slicing direction arrow from origin

Shows the effective slicing direction (gantry normal) as a pink
arrow from the origin. Shorter and wider than the gravity arrow.
Direction: R(+alpha, X) * Z = (0, -sin(a), cos(a)), which is
the layer stacking direction in the original mesh frame.

Fix slicing arrow visibility and add raw G-code toggle

- Disable depth test for pink slicing arrow so it renders on top of
  the tilted bed geometry (was being occluded)
- Remove unnecessary 5mm Z-offset from arrow position
- Add m_belt_show_raw toggle to GCodeViewer
- Add "Show raw G-code (slicing frame)" checkbox in legend when
  belt mode is active

Implement to_machine_coords inverse rotation for belt printer G-code

The slicing pipeline rotates the mesh by R(-alpha, X) and shifts Z to
start at 0. The G-code output now undoes this transform via
to_machine_coords: R(+alpha, X) * T(0,0,+z_shift), recovering the
original machine-frame coordinates where Y is horizontal and Z is
vertical.

Changes:
- GCodeWriter: implement to_machine_coords with inverse rotation + Z-shift
- GCodeWriter: add belt_z_shift member and setter/getter
- GCode.cpp: compute Z-shift from print objects (same logic as
  PrintObjectSlice) and pass to writer; write z_shift to G-code header
- GCodeProcessor: parse belt_z_shift from G-code header
- GCodeViewer: store belt_z_shift from processor result

Wire raw G-code toggle to apply slicing-frame view transform

When "Show raw G-code (slicing frame)" is checked in the preview
legend, the view matrix is modified to apply R(-alpha, X) * T(0,0,-z_shift)
to the toolpath rendering. This shows the G-code as it was during
slicing: rotated part with horizontal layers.

Default (unchecked): machine-frame view — upright part with tilted layers.

Remove belt printer placeholder comment from GCodeProcessor

The preview now correctly displays machine-frame G-code with the
optional raw view toggle. No transform is needed in the processor.

src/libslic3r/PrintObject.cpp

@@ -3392,7 +3392,13 @@ void PrintObject::update_slicing_parameters()
     // Orca: updated function call for XYZ shrinkage compensation
     if (!m_slicing_params.valid) {
           coordf_t object_height = this->model_object()->max_z();
-          // Belt printer: height adjustment placeholder (to be implemented in next cycle).
+          if (this->print()->config().belt_printer.value) {
+              // After mesh rotation R(-alpha, X), effective height in the slicing
+              // direction is y_extent*sin(a) + z_extent*cos(a).
+              double angle_rad = Geometry::deg2rad(this->print()->config().belt_printer_angle.value);
+              BoundingBoxf3 bb = this->model_object()->raw_bounding_box();
+              object_height = bb.size().y() * std::sin(angle_rad) + bb.size().z() * std::cos(angle_rad);
+          }
           m_slicing_params = SlicingParameters::create_from_config(this->print()->config(), m_config, object_height,
                                                                    this->object_extruders(), this->print()->shrinkage_compensation());
       }
@@ -3432,9 +3438,14 @@ SlicingParameters PrintObject::slicing_parameters(const DynamicPrintConfig &full
     sort_remove_duplicates(object_extruders);
     //FIXME add painting extruders
 
-    if (object_max_z <= 0.f)
-        object_max_z = (float)model_object.raw_bounding_box().size().z();
-    // Belt printer: height adjustment placeholder (to be implemented in next cycle).
+    if (object_max_z <= 0.f) {
+        BoundingBoxf3 bb = model_object.raw_bounding_box();
+        object_max_z = (float)bb.size().z();
+        if (print_config.belt_printer.value) {
+            double angle_rad = Geometry::deg2rad(print_config.belt_printer_angle.value);
+            object_max_z = (float)(bb.size().y() * std::sin(angle_rad) + bb.size().z() * std::cos(angle_rad));
+        }
+    }
     return SlicingParameters::create_from_config(print_config, object_config, object_max_z, object_extruders, object_shrinkage_compensation);
 }