vesa: for jpg or png images, tile the image across the screen

vesa: for jpg or png images, tile the image across the screen

If we specify a smaller image than the screen size, tile it across the
screen rather than leaving a big black box.  This may have to be made
configurable.

Also note that Syslinux currently does not handle the case of an image
bleeding off the end of the screen.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>