ifcviewer: GPU cull fwd/rev reflection bucketing (step 3b)

Extend the GPU-cull indirect buffer from M to 2M commands: the first M
are the forward (non-reflected, CCW) bucket, the second M are the
reverse (reflected, CW) bucket.  The compact shader reads flags bit 0
from the AABB SSBO and routes each survivor to the appropriate bucket
via bucket = reflected ? mesh_id + M : mesh_id.

uploadGpuCullStaticBuffers() now precomputes exact per-mesh fwd/rev
instance counts so each bucket reserves only the slots it needs
(total visible_ssbo size unchanged — sum of fwd + rev = total).

Draw loop issues two MDIs per model under IFC_GPU_CULL: first M
commands CCW, next M commands CW.

Sub-draws doubled (172k → 345k) which further regresses FPS due to
command-processor overhead from zero-instance sub-draws — the same
issue noted in 3a.  MDI compaction remains the fix.

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

src/ifcviewer/ViewportWindow.h

@@ -108,13 +108,20 @@ struct ModelGpuData {
     // the instanceCount field of gpu_indirect_buffer is rewritten by the
     // cull shader (zeroed by the reset shader, atomically incremented as
     // survivors are appended into gpu_visible_ssbo at mesh_base[i] + local).
-    GLuint   gpu_indirect_buffer     = 0;
-    size_t   gpu_indirect_capacity   = 0;
-    GLuint   gpu_visible_ssbo        = 0;
-    size_t   gpu_visible_capacity    = 0;
-    GLuint   gpu_mesh_base_ssbo      = 0;
-    size_t   gpu_mesh_base_capacity  = 0;
-    uint32_t gpu_mesh_command_count  = 0;
+    // Layout per model:
+    //   commands[0..M)        fwd bucket (non-reflected, CCW winding)
+    //   commands[M..2M)       rev bucket (reflected, CW winding)
+    // gpu_mesh_command_count = 2M; gpu_forward_command_count = M.
+    // Each bucket gets its own mesh_base[] slot and its own visible[]
+    // range, sized to the exact per-mesh count of fwd / rev instances.
+    GLuint   gpu_indirect_buffer       = 0;
+    size_t   gpu_indirect_capacity     = 0;
+    GLuint   gpu_visible_ssbo          = 0;
+    size_t   gpu_visible_capacity      = 0;
+    GLuint   gpu_mesh_base_ssbo        = 0;
+    size_t   gpu_mesh_base_capacity    = 0;
+    uint32_t gpu_mesh_command_count    = 0;
+    uint32_t gpu_forward_command_count = 0;
 
     // Dynamic visible-instance index buffer (std430, binding = 1).
     // Re-uploaded each frame from visible_flat_.