Belt: index apron layers into the first layer's nozzle map

Apron bands looked up their filament and nozzle config slot with a running
counter, while object layers use Layer::id(), so band N read the map of
object layer N. They precede layer 0 and now use its assignment.

Raised in Hanif Koh's review of #14394.

src/libslic3r/GCode.cpp

@@ -5326,7 +5326,9 @@ LayerResult GCode::process_belt_brim_layer(
             break;
         }
 
-    m_cur_layer_idx = m_belt_brim_layer_idx ++;
+    // Apron bands precede object layer 0 and have no layer id of their own; they take the
+    // filament and nozzle assignment in effect at the first object layer.
+    m_cur_layer_idx = 0;
 
     // Publish the band's Z for _extrude()'s first-layer-plane probe, and make sure
     // it cannot leak past this layer even if an extrusion throws.