Fix Proced display header alignment under 'text-scale-adjust'

Fix Proced display header alignment under 'text-scale-adjust'

* lisp/proced.el (proced-mode): Force the header-line to scale
according to 'text-scale-adjust'.
(proced-header-line): Use width of the header-line's face's
characters as align-to units, not the width of the frame's
default face's font.  (Bug#64752)

* src/xdisp.c (calc_pixel_width_or_height): Use font's
average_width or space_width, not max_width, as the correct
measure of the font's width.  This is consistent with the rest of
the display engine.