Add WMO portal culling infrastructure and fix single-player character flow

Portal-based visibility culling for WMO rendering (disabled by default,
needs debugging for complex WMOs like Stormwind). Skip character creation
screen when characters already exist in single-player mode.

src/rendering/performance_hud.cpp

@@ -370,6 +370,9 @@ void PerformanceHUD::render(const Renderer* renderer, const Camera* camera) {
             ImGui::Text("Instances: %u", wmoRenderer->getInstanceCount());
             ImGui::Text("Triangles: %u", wmoRenderer->getTotalTriangleCount());
             ImGui::Text("Draw Calls: %u", wmoRenderer->getDrawCallCount());
+            if (wmoRenderer->isPortalCullingEnabled()) {
+                ImGui::Text("Portal Culled: %u groups", wmoRenderer->getPortalCulledGroups());
+            }
 
             ImGui::Spacing();
         }