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/GCode/GCodeProcessor.hpp

@@ -227,6 +227,8 @@ class Print;
         bool support_traditional_timelapse{true};
         float printable_height;
         float z_offset;
+        // Belt printer: angle for coordinate transformation in preview.
+        float belt_printer_angle{ 0.f };
         SettingsIds settings_ids;
         size_t filaments_count;
         bool backtrace_enabled;