patch for some acme scrolling issues that've been bugging me
for years.

1) when middle-button scrolling to the bottom of a long
document, acme doesn't go right to the end - there's a bit
more of the document left below.

2) acme constrains the mouse to remain within the boundaries
of the scrollbar. i find this unnecessary and sometimes unhelpful.

3) clicking with b1 or b3 on the very top line's-worth
of the scrollbar does nothing, meaning that it's essentially
dead space, and also that it's awkward to scroll
very small (one or two line) windows.

one gratuitous change: a piece of code from optionally
using fixed-point arithmetic to use a vlong;
it looks simpler that way and the overhead
is negligible by my reckoning.

YMMV.