cache drawn entries in draw_menu

[Jason Butler]

I was speaking with phcoder on IRC about gfxmenu/gui_list.c finction draw_menu. 
He says it should only redraw 2 entries and cahe drawn entries, and that most 
drawings should be done to a separate target.

To implement this I think I need more information.

draw_menu uses the function grub_video_blit_bitmap to output the entry icon and 
title to the video adapter. Which entries should be cached and which redrawn? 
What target should most drawing be done to? If its not the video adapter then 
wont it not be displayed?