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

[Carlos Pita]

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. It seems to me that the obvious candidate to host
that code is define_fringe_bitmap. The once_p clause is related to
pdumper which I ignore all about.

Best regards
--
Carlos