ifcviewer: MDI compaction via glMultiDrawElementsIndirectCount

Pack compute shader compacts non-empty indirect commands into
contiguous fwd/rev ranges, eliminating ~690k empty sub-draws that
dominated command-processor overhead.  GL 4.6 entrypoint loaded via
getProcAddress with ARB fallback; graceful degradation to uncompacted
MDI when unavailable.

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

src/ifcviewer/README.md

@@ -809,9 +809,12 @@ single giant model / <18 cores  CPU BVH trv             Phase 3E GPU cull (plann
             Occlusion halves survivors on dense interiors.  Depth
             pre-pass has same CP overhead as color pass (690k empty
             sub-draws); cost dominated by MDI command processing.
-      - [ ] MDI compaction — compact non-empty commands into contiguous
-            buffer, use `glMultiDrawElementsIndirectCount` (GL 4.6 /
-            `ARB_indirect_parameters`).  Deferred until all feature buckets
-            land so we can introduce GL 4.6 loading once, cleanly.
+      - [x] MDI compaction — pack compute shader compacts non-empty
+            commands into contiguous fwd/rev ranges; drawn via
+            `glMultiDrawElementsIndirectCount` (GL 4.6 /
+            `ARB_indirect_parameters`, loaded via `getProcAddress`).
+            Eliminates ~690k empty sub-draws from both depth pre-pass
+            and color pass.  Falls back to uncompacted MDI when the
+            entrypoint is unavailable.
 - [ ] Vulkan/MoltenVK backend for macOS
 - [ ] Embedded Python scripting console