Got rid of segfault when uploading VBOs.

Added go-to-zero function.
Reworked how work pos is obtained.

grbl_machine.h

@@ -165,7 +165,7 @@ struct machine_state_init : public machine_state {
     init_stage init_state = init_stage::start;
     void move_to_next_init_stage();
 
-    machine* cnc;
+    machine *cnc;
 };
 
 struct machine_state_idle : public machine_state {
@@ -203,7 +203,7 @@ struct machine_state_run_program : public machine_state {
     bool continue_program();
     void move_to_next_run_stage();
 
-    machine* cnc;
+    machine *cnc;
     run_stage stage = run_stage::start;
     bool run_failed;
     size_t run_error;
@@ -228,7 +228,11 @@ struct machine : public transport_callbacks {
 
     void set_listener(grbl::machine_listener *listener);
     void connect();
-    void run_program(const grbl::program& pgm, std::string work_offset);
+
+    void set_work_offset(std::string work_offset);
+    std::array<float, 3> get_work_pos() const;
+
+    void run_program(const grbl::program& pgm, const std::string& work_offset);
     void check_program(const grbl::program& pgm);
     void request_jog(jog_state jog) const;
     void cancel_jog() const;
@@ -246,6 +250,8 @@ struct machine : public transport_callbacks {
     // 0 = G54, 1 = G55, etc
     void zero_offset(int which);
 
+    void go_to_zero(bool x, bool y, bool z);
+
     // 0 = G54, 1 = G55, etc axis 0=X, 1=Y, 2=Z
     void zero_offset_axis(int offset_index, int axis);
 
@@ -266,6 +272,12 @@ protected:
     std::map<std::string, std::string, settings_cmp> settings;
     std::map<std::string, std::string, parameters_cmp> parameters;
 
+    std::string current_work_offset;
+    float current_work_offset_values[3] = {0};
+public:
+    const float *get_current_work_offset_values() const;
+protected:
+
     volatile size_t awaiting_responses = 0;
     realtime_status_report last_report{};
     machine_listener *listener = nullptr;