acme: implement position-dependent scroll-wheel scrolling

acme: implement position-dependent scroll-wheel scrolling

This brings acme scrolling behaviour in line with that of 9front's rio
and sam, where the amount scrolled varies with a vertical position of
the pointer within the window, similar to how the scrollbar works.

At some point it would be good to implement a line-at-a-time scrolling
when the Shift key is pressed, as seen in rio.  For this to happen the
acme keyboard input needs to be rewritten in terms of /dev/kbd instead
of relying on keyboard(2) -- that is, the /dev/cons interface.