Fix duplicate G29 preparation time (#15274)

The time estimate added 260 s for every G29 on non-Bambu printers. Prusa MK4, MK4S and CORE One start G-code splits bed leveling across six G29 commands, so those printers showed about 26 extra minutes. The 260 s is now added once per G-code processing run. Bambu printers still only count a G29 inside an M622 J1 block, and their estimates are unchanged.

Fixes #15088

src/libslic3r/GCode/GCodeProcessor.hpp

@@ -1280,6 +1280,7 @@ inline constexpr float DEFAULT_FILAMENT_DENSITY = 1.245f;
         bool m_detect_layer_based_on_tag {false};
         int m_seams_count;
         bool m_measure_g29_time {false};
+        bool m_g29_time_added {false};
         bool m_single_extruder_multi_material;
         float m_preheat_time;
         int m_preheat_steps;