[RFC] Introduce an API to add frontend-specific data to savestates

[jgcodes2020]

This might be useful for several things:

• For frontends implementing input recording, savestates can store input data and an associated input file, allowing the recovery of an input sequence that was previously overwritten
• Frontends can store an image of the framebuffer, and present that image when they load a savestate.

Proposed API:

typedef struct {
  void* cb_data;

  void (*save_extra_data)(void* context, const void** data_out, unsigned int* len_out);
  void (*load_extra_data)(void* context, void* data, unsigned int len);
} m64p_save_data_handler;