Test to determine character left-overhangs a tab stretch.

[Keith David Bershatsky]

I am working on feature requests #22873 (multiple fake cursors); and, #17684 
(crosshairs).

Fake cursors are placed on a line in the order of left to right.  
ns/x_draw_window_cursor calls draw_phys_cursor_glyph, which calls draw_glyphs.

Certain wide characters to the immediate right of a tab stretch cause the tab 
stretch to be redrawn -- erasing the fake cursors that were previously placed 
along the tab stretch.  Here is a screenshot depicting a tab stretch after the 
cursors have been erased from it -- the tab stretch had red hbar cursors before 
they were erased:

https://www.lawlist.com/images/glyph_overhang_001.png

How can programmatically test for this situation?

Thanks,

Keith