map smaller res onto current frambuffer

[CarlFK]

my box's post is 640x480. it would be nice if that was piped though to the display, even if the display is 1280x720. I dobn't care if it is scaled up to fill the screen - being able to see a small version in a corner or centred would be fine.

mithro: Have to reset the data pointer after each line rather than after each frame

for larger images, discard what doesn't fit.