bug#37755: Logic in init_fringe_bitmap should be moved to backends (maybe rif->define_fringe_bitmap)

[Eli Zaretskii]

> From: Carlos Pita <carlosjosepita@gmail.com>
> Date: Mon, 14 Oct 2019 23:30:08 -0300
> 
> In fringe.c you have init_fringe_bitmap with this structure:
> 
> ```
> #if defined (HAVE_X_WINDOWS)
> ...
> #ifdef USE_CAIRO
> ...
> #endif
> ...
> #endif
> #ifdef HAVE_NTGUI
> ...
> #endif
>   if (!once_p)
>     {
>     ....
>     rif->define_fringe_bitmap (...)
>     ....
>     }
> ```
> 
> Now, this is backend specific code that should be moved behind the
> redisplay_interface.

Yes, it should.

> It seems to me that the obvious candidate to host that code is
> define_fringe_bitmap.

No, I think we need another interface, as define_fringe_bitmap is used
directly from other places.