Update nuklear_sdl_gl3.h

demo/sdl_opengl3/nuklear_sdl_gl3.h

@@ -326,6 +326,7 @@ nk_sdl_init(SDL_Window *win)
     sdl.ctx.clip.paste = nk_sdl_clipboard_paste;
     sdl.ctx.clip.userdata = nk_handle_ptr(0);
     nk_sdl_device_create();
+    sdl.time_of_last_frame = ((float)SDL_GetTicks64()) / 1000;
     return &sdl.ctx;
 }