viewport: wire up the backface-culling setting

The "Backface Culling" checkbox persisted a value and reflected it, but
nothing consumed AppSettings::backfaceCulling — the opaque pipeline
hardcoded cullMode = Back, so toggling had no effect.

Build a second opaque pipeline (cullMode None) alongside the culled one and
pick between them per-frame from a backface_culling_ flag; setBackfaceCulling
flips the flag and requests a redraw (no rebuild). ViewportWindow forwards
it, and MainWindow applies the persisted value at startup and re-applies on
change — same wiring as the nav preset.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ifcviewer/ViewportWindow.h

@@ -247,6 +247,7 @@ public:
     // Sources the shared binding table from ViewportCore; called from init
     // (env / persisted setting) and live from the Settings dialog.
     void applyNavPreset(const char* name);
+    void setBackfaceCulling(bool enabled);
 
 
     // Queue a one-shot framebuffer capture: the next rendered frame is