Fixed: Make ScrollDown escape respect margins

SD sequence (`${CSI}${N}T`) was scrolling the whole width
 of the terminal instead of just between the margins.
RI sequence (`${ESC}M`, move cursor up 1 line) was doing the same.
Fixed that.

Fixes #2576 where in tmux scrolling one of several
 side-by-side panels down resulted in all visually scrolling.