3D viewer, minor fix: vias not fully displayed when the top layer was not shown.

From master branch.

3d-viewer/3d_canvas/create_layer_items.cpp

@@ -185,8 +185,12 @@ void BOARD_ADAPTER::createLayers( REPORTER* aStatusReporter )
 
     for( PCB_TRACK* track : m_board->Tracks() )
     {
-        if( !Is3dLayerEnabled( track->GetLayer(), visibilityFlags ) ) // Skip non enabled layers
+        // Skip tracks (not vias that are on more than one layer ) on disabled layers
+        if( !track->Type() == PCB_VIA_T
+            && !Is3dLayerEnabled( track->GetLayer(), visibilityFlags ) )
+        {
             continue;
+        }
 
         // Note: a PCB_TRACK holds normal segment tracks and also vias circles (that have also
         // drill values)