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.cpp

@@ -3051,14 +3051,6 @@ void GCodeProcessor::process_tags(const std::string_view comment, bool producers
         } catch (...) {}
         return;
     }
-    // Belt printer Z-shift for raw G-code toggle in preview.
-    if (boost::starts_with(comment, " belt_z_shift = ")) {
-        try {
-            m_result.belt_z_shift = std::stof(std::string(comment.substr(16)));
-        } catch (...) {}
-        return;
-    }
-
     // wipe start tag
     if (boost::starts_with(comment, reserved_tag(ETags::Wipe_Start))) {
         m_wiping = true;