ENH: add layer progress into gcode file

As title

Signed-off-by: salt.wei <salt.wei@bambulab.com>
Change-Id: I7b083e15e0f9030d7822494c4d6fc9cc72335ec2

src/libslic3r/GCode.cpp

@@ -1335,6 +1335,8 @@ void GCode::_do_export(Print& print, GCodeOutputStream &file, ThumbnailsGenerato
     file.write_format("; %s\n", Slic3r::header_slic3r_generated().c_str());
     //BBS: total estimated printing time
     file.write_format(";%s\n", GCodeProcessor::reserved_tag(GCodeProcessor::ETags::Estimated_Printing_Time_Placeholder).c_str());
+    //BBS: total layer number
+    file.write_format("; total layer number: %d\n", m_layer_count);
     file.write_format("; HEADER_BLOCK_END\n\n");
 
     //BBS: write global config at the beginning of gcode file because printer need these config information