Centre the background picture on the console, to give a more
consistent result when the aspect ratio does not match the requested
width and height.

Once drawn for the first time, nothing should ever overwrite the
margins of the display.  We can therefore eliminate the logic used to
redraw only the margin areas, and use much simpler code to draw the
complete initial background image.

Simplify the redrawing logic further by making the background picture
buffer equal in size to the frame buffer.  In the common case of a
background picture which is designed to fill the screen, this wastes
no extra memory, and the combined code simplifications reduce the size
of fbcon.o by approximately 15%.

Redefine the concept of "margin" to match the intuitive definition
(i.e. the size of the gap, rather than the position of the boundary
line).

Signed-off-by: Michael Brown <mcb30@ipxe.org>