Move cursor to the leftmost column of window when scrolling if left margin is used.

[ttdoda]

If cursor is in the outside of margin, VT525 and xterm ignore some control sequences (e.g. IL, DL).

Following setting enables to use right and left margin when scrolling.

let &t_ti .= "\e[?69h"
let &t_te .= "\e7\e[?69l\e8"
let &t_CV = "\e[%i%p1%d;%p2%ds"

But xterm (and VT525) does not scroll correctly if window is not leftmost in screen.
This patch fixes this problem.

[codecov-io]

Codecov Report

Merging #3016 into master will increase coverage by <.01%.
The diff coverage is 46.66%.

@@            Coverage Diff             @@
##           master    #3016      +/-   ##
==========================================
+ Coverage   75.84%   75.85%   +<.01%     
==========================================
  Files          92       92              
  Lines      135295   135301       +6     
==========================================
+ Hits       102620   102633      +13     
+ Misses      32675    32668       -7

Impacted Files	Coverage Δ
src/screen.c	76.66% <46.66%> (-0.02%)	⬇️
src/if_xcmdsrv.c	84.35% <0%> (-1.08%)	⬇️
src/terminal.c	72.74% <0%> (-0.17%)	⬇️
src/channel.c	83.16% <0%> (-0.08%)	⬇️
src/os_unix.c	54.38% <0%> (-0.05%)	⬇️
src/ex_docmd.c	77.05% <0%> (-0.02%)	⬇️
src/window.c	82.22% <0%> (+0.03%)	⬆️
src/eval.c	81.52% <0%> (+0.04%)	⬆️
src/gui_gtk_x11.c	47.98% <0%> (+0.09%)	⬆️
src/term.c	59.94% <0%> (+0.15%)	⬆️
... and 3 more

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