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.
terminal-left: terminal window's left position
terminal-top: terminal window's top position
terminal-width: terminal window's width
terminal-height: terminal window's height
* grub-core/gfxmenu/theme-loader.c: Likewise.
* include/grub/gfxmenu_view.h: Likewise.
* po/exlude.pot: Likewise.
* grub-core/gfxmenu/view.c: Likewise.
Also updated minimal window size.
Also terminal_sanity_check function has been introduced.