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.

src/libslic3r/GCode.hpp

@@ -843,9 +843,6 @@ protected:
     // _extrude() needs for the first-layer-plane probe is published here instead.
     // Scoped by BeltBrimZGuard in process_belt_brim_layer(), never left set.
     std::optional<coordf_t> m_belt_brim_z;
-    // Counter standing in for Layer::id() on apron layers, which precede layer 0.
-    size_t m_belt_brim_layer_idx{0};
-
     // Belt brim only.  Brim and coincident apron bands are emitted before m_layer
     // is switched to their object, so belt_height_above_floor() would otherwise
     // read the previously visited object's belt description -- making a brim's