3D Viewer: Fix missing barrel walls for NPTH holes in OpenGL mode

NPTH holes were not showing barrel walls because their polygons were
stored in m_NPTH_ODPolys but the barrel wall generation in generateHoles()
only used m_TH_ODPolys. Add NPTH polygons to the outer polygon set before
generating through-hole geometry.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/21181

3d-viewer/3d_rendering/opengl/create_scene.cpp

@@ -754,6 +754,10 @@ void RENDER_3D_OPENGL::reload( REPORTER* aStatusReporter, REPORTER* aWarningRepo
 
     SHAPE_POLY_SET outerPolyTHT = m_boardAdapter.GetTH_ODPolys().CloneDropTriangulation();
 
+    // Include NPTH polygons so their barrel walls are also generated
+    if( m_boardAdapter.GetNPTH_ODPolys().OutlineCount() > 0 )
+        outerPolyTHT.BooleanAdd( m_boardAdapter.GetNPTH_ODPolys() );
+
     outerPolyTHT.BooleanIntersection( m_boardAdapter.GetBoardPoly() );
 
     m_outerThroughHoles = generateHoles( m_boardAdapter.GetTH_ODs().GetList(), outerPolyTHT,