Add belt printer transform pipeline: slicing rotation, G-code coords, preview

- Implement core belt slicing pipeline: R(-alpha, X) mesh rotation in PrintObjectSlice with corrected object height calculation for proper layer count
Add to_machine_coords() in GCodeWriter to convert slicing-frame coordinates back to machine-frame, propagated through GCode,
GCodeProcessor, and GCodeViewer
Add belt-mode UI: tilted bed visualization, slicing-direction arrow, and raw G-code toggle to switch between machine-frame and slicing-frame views

This is a combination of 6 commits.

checkpoint 1: initial MVP.  Slicing functions, but rotates instead of skews are happening and a lot of other stuff too

getting somewhere, getting to the point where I need to figure out how to verify this stuff

this appears to be a dead end.

getting somewhere I think maybe

I'm pretty sure we've completely lost the plot at this point and need to restart this process...

remove slice logic in preparation for new, more invasive plan

src/libslic3r/GCodeWriter.hpp

@@ -125,6 +125,12 @@ public:
     void set_is_first_layer(bool bval) { m_is_first_layer = bval; }
     GCodeFlavor get_gcode_flavor() const { return config.gcode_flavor; }
 
+    // Belt printer: set the belt angle and precompute sin/cos for coordinate transformation.
+    void set_belt_angle(double angle_deg);
+    bool is_belt_printer() const { return m_belt_angle_rad != 0.; }
+    // Transform a point from the slicing frame to machine/world coordinates (inverse shear).
+    Vec3d to_machine_coords(const Vec3d &pos) const;
+
     // Returns whether this flavor supports separate print and travel acceleration.
     static bool supports_separate_travel_acceleration(GCodeFlavor flavor);
   private:
@@ -177,6 +183,11 @@ public:
 
     //SoftFever
     bool            m_is_bbl_printers = false;
+
+    // Belt printer coordinate transformation (YZ shear)
+    double          m_belt_angle_rad = 0.;
+    double          m_belt_cos = 1.0;
+    double          m_belt_sin = 0.0;
     double          m_current_speed;
     bool            m_is_first_layer = true;