`retro_frame_time_callback_t` does not act like document

I'm writing a core with libretro and get a deadlock following the document/header file. After doing some trace I found `retro_frame_time_callback_t` is not called before first `retro_run()`.

https://github.com/libretro/libretro-common/blob/2aab9683c541db49b3aca0ece66b5a7e545697b9/include/libretro.h#L2635

> It will be called right before retro_run() every frame.