* grub-core/gfxmenu/theme_loader.c: New global options for the

   * grub-core/gfxmenu/theme_loader.c: New global options for the
       theme background image handling. desktop-image-scale-method,
       desktop-image-h-align, desktop-image-v-align.
       * grub-core/gfxmenu/view.c: Likewise.
       * include/gfxmenu_view.h: Likewise.
       * include/bitmap_scale.h: Proportional scale functions introduced.
       * grub-core/video/bitmap_scale.c: Likewise. Verification checks are
       put in a separate functions. GRUB_ERR_BUG is set for grub_error in
       cases of unexpected input variables for scale functions.
       * docs/grub.texi: Updated documentation for new options.