Part 2.5: Add global shear transform, support clipping, and belt UI improvements

- Implement per-object global shear transform in PrintObject with
  layer Z-offset calculation, config invalidation, and fix for
  shared-object layer optimization breaking copied objects
- Clip support layers to the transformed belt floor plane and begin
  work on tree support adaptation for sheared coordinate space
- Improve belt UI: gray out inactive sub-options, add B keyboard
  shortcut for G-code viewer design-view toggle, fix mesh clipping
  through build plate after shear/scale transform

y' = y + z·cot(α),
  while x' = x and z' = z

getting closer to customizable variant

getting closer

X/Y/Z shear initial

clean up UI

add 1/sin(a) transform, idea taken from blackbelt cura plugin

Things work now (turns out I've been using the wrong set of  transforms)

src/libslic3r/GCode/GCodeProcessor.hpp

@@ -227,9 +227,8 @@ class Print;
         bool support_traditional_timelapse{true};
         float printable_height;
         float z_offset;
-        // Belt printer: angle and Z-shift for coordinate transformation in preview.
+        // Belt printer: angle for coordinate transformation in preview.
         float belt_printer_angle{ 0.f };
-        float belt_z_shift{ 0.f };
         SettingsIds settings_ids;
         size_t filaments_count;
         bool backtrace_enabled;