Fix hscrolling of :align-to when display-line-numbers is in effect

Fix hscrolling of :align-to when display-line-numbers is in effect

* src/dispextern.h (struct it): Rename 'tab_offset' member to
'stretch_adjust'.
* src/xdisp.c (gui_produce_glyphs, produce_stretch_glyph)
(display_line): All users of 'tab_offset' changed.
(produce_stretch_glyph): Fix calculation of ':align-to' when
line numbers are displayed and the window is hscrolled.
(calc_pixel_width_or_height): Fix calculation of width of 'space'
display property when 'display-line-numbers' is turned on, but the
line number was not yet produced for the current glyph row.
(Bug#56176)