gfxmenu/view: Resolve false grub_errno disrupting boot process

gfxmenu/view: Resolve false grub_errno disrupting boot process

When enabling gfxmenu and choosing to boot the Xen hypervisor from its
menu, an error occurred:

  error: ../../grub-core/video/bitmap_scale.c:42:null src bitmap in grub_video_create_scaled.

The error is returned by grub_video_bitmap_create_scaled() when the
source pixmap is not there. The init_background() uses it to scale up
the background image so it can fully fit into the screen resolution.

However not all backgrounds are set by a image, i.e. the "desktop-image"
property of the theme file. Instead a color code may be used, for
example OpenSUSE's green background uses "desktop-color" property:

  desktop-color: "#0D202F"

So it is absolutely fine to call init_background() without a raw pixmap
if color code is used. A missing check has to be added to ensure the
grub_errno will not be erroneously set and gets in the way of ensuing
boot process.

The reason it happens sporadically is due to grub_errno is reset to
GRUB_ERR_NONE in other places if a function's error return can be
ignored. In particular this hunk in grub_gfxmenu_create_box() does the
majority of the reset of grub_errno returned by init_background(), but
the path may not be always chosen.

  grub_video_bitmap_load (&box->raw_pixmaps[i], path);
  grub_free (path);

  /* Ignore missing pixmaps.  */
  grub_errno = GRUB_ERR_NONE;

In any case, we cannot account on such random behavior and should only
return grub_errno if it is justified.

On the occasion move the grub_video_bitmap struct definition to the
beginning of the function.

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>