Fix TAB display when the line-number face uses a smaller/larger font

Fix TAB display when the line-number face uses a smaller/larger font

* src/dispextern.h (struct it): New member lnum_pixel_width.
* src/xdisp.c (maybe_produce_line_number): Compute the width of
the line-number display in pixels.
(x_produce_glyphs): Use it->lnum_pixel_width instead of a kludge
that used it->lnum_width and made assumptions about pixel width.