log viewer: add heuristic for scroll-direction dependent ratio distribution

log viewer: add heuristic for scroll-direction dependent ratio distribution

if the user scrolls down make 2/3 of the buffer load the downward
(newer) buffer and only 1/3 the upward (older), and vice versa, if
the user scrolls up load 2/3 of the older messages vs. 1/3 of newer
ones.

If the user scrolls around frantically we're roughly as good as
previously and in all other cases we're better now.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>