Two-sided lighting, rename misleading draw-count stat

Two bugs conflated as "weird colors":

1. Two-sided lighting.  IFC placements often embed reflection
   matrices (mirrored families).  Transforming a_normal by
   mat3(inst.transform) produces a normal pointing the wrong way
   on those instances, and max(n·L, 0) then clamps the surface to
   pure ambient — reads as dark / washed out.  Use gl_FrontFacing
   to flip n in the fragment shader so both winding orientations
   shade correctly.  The proper fix (ship an inverse-transpose
   normal matrix or a det-sign bit per instance) is still owed;
   that would unlock re-enabling GL_CULL_FACE for a big fragment-
   work win on closed solids.

2. Stats label "inst_draws" was counting indirect sub-draws, not
   actual GL draw calls — misleading since MDI collapses N sub-
   draws into one glMultiDrawElementsIndirect.  Split into
   gl_draw_calls (real GL calls, = drawn-model count) and
   indirect_sub_draws (packed sub-commands).  For a BIM model
   with 47k unique meshes at full view this now correctly reads
   "1 gl_draws (47092 sub)" rather than suggesting 47k driver
   dispatches.

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

src/ifcviewer/MainWindow.cpp

@@ -41,13 +41,15 @@ MainWindow::MainWindow(QWidget* parent)
     connect(viewport_, &ViewportWindow::frameStatsUpdated, this, [this](const ViewportWindow::FrameStats& s) {
         if (!stats_label_->isVisible()) return;
         stats_label_->setText(
-            QString("%1 fps | %2 ms | %3/%4 obj | %5/%6 tri")
+            QString("%1 fps | %2 ms | %3/%4 obj | %5/%6 tri | %7 gl_draws (%8 sub)")
                 .arg(s.fps, 0, 'f', 1)
                 .arg(s.frame_time_ms, 0, 'f', 1)
                 .arg(s.visible_objects)
                 .arg(s.total_objects)
                 .arg(s.visible_triangles)
-                .arg(s.total_triangles));
+                .arg(s.total_triangles)
+                .arg(s.gl_draw_calls)
+                .arg(s.indirect_sub_draws));
     });
 
     connect(&AppSettings::instance(), &AppSettings::showStatsChanged, this, [this](bool show) {

src/ifcviewer/ViewportWindow.cpp

@@ -123,7 +123,13 @@ uniform vec3 u_light_dir;
 out vec4 frag_color;
 
 void main() {
+    // Two-sided lighting: IFC placements frequently embed reflections
+    // (mirrored families), which flip triangle winding and invert the
+    // transformed normal.  Taking abs(dot) — or equivalently flipping n
+    // based on gl_FrontFacing — makes both sides shade correctly
+    // regardless of winding / reflection state.
     vec3 n = normalize(v_normal);
+    if (!gl_FrontFacing) n = -n;
     float ndotl = max(dot(n, u_light_dir), 0.0);
     float ambient = 0.25;
     float diffuse = 0.75 * ndotl;
@@ -906,7 +912,8 @@ void ViewportWindow::render() {
 
     visible_triangles_ = 0;
     visible_objects_ = 0;
-    instanced_draws_ = 0;
+    gl_draw_calls_ = 0;
+    indirect_sub_draws_ = 0;
 
     for (auto& [model_id, m] : models_gpu_) {
         if (m.hidden || !m.ssbo || m.ssbo_instance_count == 0) continue;
@@ -926,7 +933,8 @@ void ViewportWindow::render() {
             visible_triangles_ += (cmd.count / 3) * cmd.instanceCount;
             visible_objects_   += cmd.instanceCount;
         }
-        instanced_draws_ += m.indirect_command_count;
+        indirect_sub_draws_ += m.indirect_command_count;
+        ++gl_draw_calls_;
     }
     gl_->glBindBuffer(GL_DRAW_INDIRECT_BUFFER, 0);
 
@@ -964,16 +972,17 @@ void ViewportWindow::render() {
         stats.total_triangles = total_tri;
         stats.visible_triangles = visible_triangles_;
         stats.unique_meshes = total_meshes;
-        stats.instanced_draws = instanced_draws_;
+        stats.gl_draw_calls = gl_draw_calls_;
+        stats.indirect_sub_draws = indirect_sub_draws_;
         emit frameStatsUpdated(stats);
 
         qDebug("[frame] %.1f fps  %.2f ms  obj %u/%u  tri %u/%u  "
-               "meshes %u  inst_draws %u  "
+               "meshes %u  gl_draws %u  sub_draws %u  "
                "vram %.1f MB (vbo %.1f + ebo %.1f + ssbo %.1f)  models %zu (%zu hidden)",
                last_fps_, 1000.0f / last_fps_,
                visible_objects_, total_obj,
                visible_triangles_, total_tri,
-               total_meshes, instanced_draws_,
+               total_meshes, gl_draw_calls_, indirect_sub_draws_,
                (total_vbo + total_ebo + total_ssbo) / (1024.0*1024.0),
                total_vbo / (1024.0*1024.0),
                total_ebo / (1024.0*1024.0),

src/ifcviewer/ViewportWindow.h

@@ -136,7 +136,8 @@ public:
         uint32_t total_triangles;
         uint32_t visible_triangles;
         uint32_t unique_meshes;
-        uint32_t instanced_draws;
+        uint32_t gl_draw_calls;        // actual glMultiDrawElementsIndirect issues per frame
+        uint32_t indirect_sub_draws;   // total commands packed into those indirect buffers
     };
 
 signals:
@@ -202,7 +203,8 @@ private:
     // Per-frame stats
     uint32_t visible_triangles_ = 0;
     uint32_t visible_objects_ = 0;
-    uint32_t instanced_draws_ = 0;
+    uint32_t gl_draw_calls_ = 0;
+    uint32_t indirect_sub_draws_ = 0;
 
     // Reused scratch: visible-instance index lists per mesh, flattened into
     // `visible_flat_` for upload.  Both live in the parent object to avoid