(compute_motion): Use actual window width if WIDTH is -1,

(compute_motion): Use actual window width if WIDTH is -1,
properly accounting for continuation glyph on non-window systems.
(Fcompute_motion): Use actual window width if WIDTH is nil, and
actual window width/height if TOPOS is nil, properly accounting for
continuation glyphs on non-window systems, and optional header lines.
(vmotion): Let compute_motion calculate actual window width.