Fix SPE-2650: Respect z offset for wipe tower first layer

Wipe tower bases its position on current writer position.
It was not properly set on the first layer.

src/libslic3r/GCode.cpp

@@ -2994,6 +2994,10 @@ std::string GCodeGenerator::change_layer(
             Vec3d position{this->writer().get_position()};
             position.z() = print_z;
             this->writer().update_position(position);
+        } else {
+            Vec3d position{this->writer().get_position()};
+            position.z() = position.z() + m_config.z_offset;
+            this->writer().update_position(position);
         }
     }