fix: Regression setcellwidths with GUI

[h-east]

A regression has occurred since version 9.1.0344.
(ID-Call as patch authors: @mikoto2000 @zeertzjq)

Related: #14539 #14540

Step to repro.

1. Start GUI Vim with few parameters.

$ vim -g -Nu NONE +'call setcellwidths([[0x2502, 0x2502, 2]])' +'call setline(1, ["abc\u2502def"])'

2. Behavior of GUI Vim 9.1.0344
   There is an extra space before d.
   

   After type fd, The cursor will move to the extra space and d will appear.
   

Expected behavior

3. Behavior of GUI Vim before version 9.1.0343 or with this PR patch applied.
   There is no extra space before d.
   

   After type fd.
   

[chrisbra]

thanks. Can you please turn that into a regression test?

[h-east]

@chrisbra This problem only occurs in the GUI, testing using VerifyScreenDump() (term_dumpdiff()) will not show the difference before and after the fix.
Is there any better way?

[chrisbra]

no sure, okay let me include it then.

[yegappan]

@chrisbra This problem only occurs in the GUI, testing using VerifyScreenDump() (term_dumpdiff()) will not show the difference before and after the fix. Is there any better way?

Sometime ago I added the test_gui_event() function to test GUI specific events. We should
add a similar function to test GUI specific rendering of characters.