GFXMENU: Allow for the use of a background image in the terminal window

[Towheed Mohammed]

Because GRUB_THEME takes precedence over GRUB_BACKGROUND, the terminal-window 
never shows a background image.  Could we allow for the use of a background 
image in the terminal-window?  This will certainly make a theme more complete.

I can think of two ways to do this:
1.  Remove the precedence of GRUB_THEME over GRUB_BACKGROUND in 00_header and 
patch the code to scale the image properly to flt within the center slice of 
the terminal window.
2.  Allow the center slice to be used as a background image.  Presently, the 
image for the center slice is replaced by a black background.

Thank you.
Towheed Mohammed