Obtain X / HPOS with move_it_to at eol when buffer-display-table line-feed

[Keith David Bershatsky]

I am working on my own feature requests 17684 (thin vertical line) and 22873 
(multiple fake cursors).

I am having difficulty calculating the X and HPOS using move_it_to when a 
buffer-display-table contains an entry for a line feed (aka \n).  I am looking 
for a more eloquent way of of handling this instead of subtracting the 
frame-char-width from it.current.pos, and also subtracting 1 from 
it.current.hpos.  We can test the buffer-display-table with something like 
(aref buffer-display-table ?\n), which in my use-case yields [(182 . 127) 10] 
because I also use face to color the eol character.

We test for a line feed with:  FETCH_BYTE (IT_BYTEPOS (it)) == '\n'

We move to POS using:

move_it_to (&it, POS, it.last_visible_x, it.last_visible_y - 1, -1, MOVE_TO_POS 
| MOVE_TO_X | MOVE_TO_Y);

Is there a better way to handle this instead of subtracting frame-char-width 
from X and subtracting 1 from HPOS?

Thanks,

Keith