Backface culling with reflection-aware two-pass MDI

Enables GL_CULL_FACE by default (user-toggleable in Settings) so
closed solids skip shading their back halves.  The catch is that
IFC placements can contain reflections (mat4 with det<0 — mirrored
families, symmetric instances).  Naively culling would make every
mirrored instance vanish because the rasterizer sees its screen-space
winding as backwards.

Fix: detect reflections at upload time via determinant sign, bucket
visible instances into forward (det>=0) and reverse (det<0) per mesh
during culling, and issue two glMultiDrawElementsIndirect calls per
model with glFrontFace toggled CCW/CW between them.  The indirect
buffer is still one buffer — just split into a forward slice followed
by a reverse slice, with m.indirect_forward_count recording the split.

Vertex shader flips the normal when the transform has negative
determinant, keeping lighting correct on mirrored instances.  The
fragment shader keeps the gl_FrontFacing fallback as a safety net
when culling is disabled (e.g. for files with open shells).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ifcviewer/AppSettings.h

@@ -37,9 +37,13 @@ public:
     bool showStats() const;
     void setShowStats(bool value);
 
+    bool backfaceCulling() const;
+    void setBackfaceCulling(bool value);
+
 signals:
     void geometryLibraryChanged(const QString& value);
     void showStatsChanged(bool value);
+    void backfaceCullingChanged(bool value);
 
 private:
     AppSettings();
@@ -48,6 +52,7 @@ private:
 
     QString geometry_library_;
     bool show_stats_ = false;
+    bool backface_culling_ = true;
 };
 
 #endif // APPSETTINGS_H