[fbcon] Avoid redrawing unchanged characters when scrolling

[fbcon] Avoid redrawing unchanged characters when scrolling

Scrolling currently involves redrawing every character cell, which can
be frustratingly slow on large framebuffer consoles.  Accelerate this
operation by skipping the redraw for any unchanged character cells.

In the common case that large areas of the screen contain whitespace,
this optimises away the vast majority of the redrawing operations.

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