add preamble to gcode processor if using start_gcode_manual

https://github.com/supermerill/SuperSlicer#1172

src/libslic3r/GCode.cpp

@@ -719,6 +719,11 @@ void GCode::do_export(Print* print, const char* path, GCodeProcessor::Result* re
     }
 
     BOOST_LOG_TRIVIAL(debug) << "Start processing gcode, " << log_memory_info();
+    //klipper can hide gcode into a macro, so add guessed init gcode to the processor.
+    if (this->config().start_gcode_manual) {
+        std::string gcode = m_writer.preamble();
+        m_processor.process_string(gcode, [print]() { print->throw_if_canceled(); });
+    }
     m_processor.process_file(path_tmp, true, [print]() { print->throw_if_canceled(); });
     DoExport::update_print_estimated_times_stats(m_processor, print->m_print_statistics);
     if (result != nullptr)

src/libslic3r/GCode/GCodeProcessor.cpp

@@ -842,6 +842,15 @@ void GCodeProcessor::reset()
     m_width_compare.reset();
 #endif // ENABLE_GCODE_VIEWER_DATA_CHECKING
 }
+void GCodeProcessor::process_string(const std::string& gcode, std::function<void()> cancel_callback)
+{
+    std::stringstream ss(gcode);
+    std::string strline;
+    while ( std::getline(ss, strline))
+        m_parser.parse_line(strline, [this](GCodeReader& reader, const GCodeReader::GCodeLine& line) {
+            process_gcode_line(line); 
+        });
+}
 
 void GCodeProcessor::process_file(const std::string& filename, bool apply_postprocess, std::function<void()> cancel_callback)
 {

src/libslic3r/GCode/GCodeProcessor.hpp

@@ -479,6 +479,7 @@ namespace Slic3r {
         // Process the gcode contained in the file with the given filename
         // throws CanceledException through print->throw_if_canceled() (sent by the caller as callback).
         void process_file(const std::string& filename, bool apply_postprocess, std::function<void()> cancel_callback = nullptr);
+        void process_string(const std::string& gcode, std::function<void()> cancel_callback = nullptr);
 
         float get_time(PrintEstimatedTimeStatistics::ETimeMode mode) const;
         std::string get_time_dhm(PrintEstimatedTimeStatistics::ETimeMode mode) const;