SPE-1739: Fixed calculation of gcode preview sequential slider endpoints

(cherry picked from commit 578921257d14e2c07611dfbb89cda5f9dc776e0c)

src/libslic3r/GCode/GCodeProcessor.hpp

@@ -787,6 +787,11 @@ class Print;
 
         // Streaming interface, for processing G-codes just generated by PrusaSlicer in a pipelined fashion.
         void initialize(const std::string& filename);
+        void initialize_result_moves() {
+            // 1st move must be a dummy move
+            assert(m_result.moves.empty());
+            m_result.moves.emplace_back(GCodeProcessorResult::MoveVertex());
+        }
         void process_buffer(const std::string& buffer);
         void finalize(bool post_process);