bug#41125: 28.0.50; Fwindow_text_pixel_size uses FETCH_CHAR (charpos)

[Pip Cet]

Fwindow_text_pixel_size contains this code:

      start = pos = BEGV;
      while ((pos++ < ZV) && (c = FETCH_CHAR (pos))
         && (c == ' ' || c == '\t' || c == '\n' || c == '\r'))
    start = pos;
      while ((pos-- > BEGV) && (c = FETCH_CHAR (pos)) && (c == ' ' ||
c == '\t'))
    start = pos;

which cannot possibly be correct: FETCH_CHAR takes a byte position,
not a character position, but BEGV and ZV are measured in characters.

(I'm familiarizing myself with the xdisp.c code, partly by replacing
bytepos/charpos pairs of variables with a combined pos_t type
variable. I think that's a good idea partly because it would prevent
precisely this kind of bug.)