Add accessor for canvas_p0_ in Canvas

src/gui/canvas.h

@@ -13,7 +13,7 @@ class Canvas {
  public:
   Canvas() = default;
   Canvas(ImVec2 canvas_size)
-      : canvas_sz_(canvas_size), custom_canvas_size_(true) {}
+      : custom_canvas_size_(true), canvas_sz_(canvas_size) {}
 
   void Update();
 
@@ -22,6 +22,8 @@ class Canvas {
   void DrawGrid();
   void DrawOverlay();  // last
 
+  auto GetZeroPoint() const { return canvas_p0_; }
+
  private:
   bool enable_grid_ = true;
   bool enable_context_menu_ = true;