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.

	Don't compile video modules on yeeloong since video subsystem is part
	of kernel.

	* conf/common.rmk (pkglib_MODULES) [yeeloong]: Remove video.mod,
	video_fb.mod, bitmap.mod, font.mod, gfxterm.mod and bufio.mod
	* conf/mips-yeeloong.rmk (kernel_img_HEADERS): Add bitmap.h,
	video.h, gfxterm.h, font.h, bitmap_scale.h and bufio.h.
	* conf/mips.rmk (kernel_img_HEADERS): Add values instead of overwriting.
	* include/grub/bitmap.h: Add EXPORT_FUNC and EXPORT_VAR.
	* include/grub/bitmap_scale.h: Likewise.
	* include/grub/bufio.h: Likewise.
	* include/grub/font.h: Likewise.
	* include/grub/gfxterm.h: Likewise.
	* include/grub/video.h: Likewise.
	* include/grub/vbe.h: Don't include video_fb.h.
	* video/i386/pc/vbe.c: Include video_fb.h.
	* commands/i386/pc/vbetest.c: Include video.h.