Add function to calculate line direction

src/window.c

@@ -72,6 +72,13 @@ void clear_window(const window *wnd, colour colour) {
 
 void swap_buffers(const window *wnd) { SDL_RenderPresent(wnd->renderer); }
 
+vec2 line_direction(const line *ln) {
+  return (vec2){
+      .x = ln->p1.x - ln->p0.x,
+      .y = ln->p1.y - ln->p0.y,
+  };
+}
+
 void draw_point(const window *wnd, point p, colour colour) {
   set_colour(wnd, colour);
   SDL_RenderDrawPoint(wnd->renderer, p.x, p.y);