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.cpp

@@ -25,6 +25,7 @@ namespace {
 constexpr const char* kGeometryLibraryKey = "geometry/library";
 constexpr const char* kGeometryLibraryDefault = "hybrid-cgal-simple-opencascade";
 constexpr const char* kShowStatsKey = "viewport/show_stats";
+constexpr const char* kBackfaceCullingKey = "viewport/backface_culling";
 }
 
 AppSettings& AppSettings::instance() {
@@ -58,14 +59,27 @@ void AppSettings::setShowStats(bool value) {
     emit showStatsChanged(value);
 }
 
+bool AppSettings::backfaceCulling() const {
+    return backface_culling_;
+}
+
+void AppSettings::setBackfaceCulling(bool value) {
+    if (backface_culling_ == value) return;
+    backface_culling_ = value;
+    persist();
+    emit backfaceCullingChanged(value);
+}
+
 void AppSettings::load() {
     QSettings settings;
     geometry_library_ = settings.value(kGeometryLibraryKey, kGeometryLibraryDefault).toString();
     show_stats_ = settings.value(kShowStatsKey, false).toBool();
+    backface_culling_ = settings.value(kBackfaceCullingKey, true).toBool();
 }
 
 void AppSettings::persist() {
     QSettings settings;
     settings.setValue(kGeometryLibraryKey, geometry_library_);
     settings.setValue(kShowStatsKey, show_stats_);
+    settings.setValue(kBackfaceCullingKey, backface_culling_);
 }