fix slow_down_layers (#5396)

Setting slow_down_layers to 2 or more results in wrong number of slowed
down layers.

Do not increment layer_id since bottom layer (#0) handled separately.

src/libslic3r/GCode.cpp

@@ -5195,7 +5195,7 @@ std::string GCode::_extrude(const ExtrusionPath &path, std::string description,
             speed = m_config.get_abs_value("initial_layer_speed");
     }
     else if(m_config.slow_down_layers > 1){
-        const auto _layer = layer_id() + 1;
+        const auto _layer = layer_id();
         if (_layer > 0 && _layer < m_config.slow_down_layers) {
             const auto first_layer_speed =
                 is_perimeter(path.role())