bug#30226: Fixing it->pixel_width / it->current_x when tabs and line numbers.

[Keith David Bershatsky]

A major break-through in the tracing of bug 30226 ....  it->current_x differs 
between the real thing (when display_line calls PRODUCE_GLYPHS), versus the 
simulation (when move_it_in_display_line_to calls PRODUCE_GLYPHS), _because_ 
maybe_produce_line_number advances it->current_x in the former but _not_ the 
latter.

In the revised fact pattern described in Message #26 of this thread (i.e., TAB 
TAB Hello-world, and w->hscroll >= 2):

https://debbugs.gnu.org/cgi/bugreport.cgi?bug=30226#26

it->current_x stops at 7 when w->hscroll == 2.

In the simulation (move_it_in_display_line_to that does _not_ produce glyphs), 
it is impossible to reach an X of 35 because that would be in the middle of the 
STRETCH.  maybe_produce_line_number does _not_ advance it->current_x in the 
simulation.

In the real thing (display_line actually produces glyphs), 
maybe_produce_line_number advances it->current_x from 7 to 35.

SOLUTION (suggested):  Revise move_it_in_display_line_to advance X by 
simulating a call to maybe_produce_line_number and permitting that function to 
advance X.