bug#35062: [PATCH v3 2/3] constify a bit of xterm.c

[Eli Zaretskii <eliz@gnu.org>]

> From: Konstantin Kharlamov <Hi-Angel@yandex.ru>
> Date: Sun,  7 Apr 2019 05:13:30 +0300
> 
> * src/xterm.c (x_cr_draw_image, x_update_begin,
> x_clear_under_internal_border, x_draw_fringe_bitmap,
> x_set_glyph_string_clipping, x_draw_glyph_string_background,
> x_draw_composite_glyph_string_foreground, x_send_scroll_bar_event): make
> code easier to follow by making explicit that some variables are
> immutable. (Bug#35062)
> ---
> v3: mention functions changed in commit messages, mention the bug
> number, and don't mention that it fixes a warning since intention  of
> changes is clear either way.

I'm really struggling with these changes.  My main problem is that I
don't see how using 'const' here improves the readability and clarity
of the code.  IMO, if the variable's name doesn't state its purpose,
adding 'const' won't help much.  And I think compilers nowadays are
smart enough to deduce this by themselves.

We had in the past similar issue with the 'register' qualifier;
nowadays we remove those whenever we change code that uses them.
Isn't it the same with 'const'?

Paul, could you please comment on these proposed changes?

Thanks.