Avoid unnecessary memcpy of whole video buffer.

	Avoid unnecessary memcpy of whole video buffer.

	* grub-core/video/fb/video_fb.c (dirty): New struct.
	(framebuffer): Add members current_dirty and previous_dirty.
	(dirty): New function.
	(grub_video_fb_fill_rect): Update dirty.
	(common_blitter): Likewise.
	(grub_video_fb_scroll): Likewise.
	(doublebuf_blit_update_screen): Copy only dirty part.
	(doublebuf_pageflipping_update_screen): Likewise.
	(grub_video_fb_doublebuf_blit_init): Init dirty.
	(doublebuf_pageflipping_init): Likewise.
	(grub_video_fb_setup): Likewise.