bug#37770: [PATCH] Expose scale factor through the redisplay interface

[Eli Zaretskii <eliz@gnu.org>]

> From: Carlos Pita <carlosjosepita@gmail.com>
> Date: Wed, 16 Oct 2019 16:08:47 -0300
> Cc: 37770@debbugs.gnu.org
> 
> Eli told me that you prefer code comments to long commit messages but
> in this case I think the rationale would be lost in fragmentary
> comments here and there. It's true that there is still the reference
> to this discussion in the commit message, but I believe it's
> convenient to quickly get a description of the change using git blame
> when browsing the code. If you disagree I will remove the notes from
> the commit message and copy them here.

The explanations should precede the file/function entries part.

> Note 1: both x_get_scale_factor and w32_get_scale_factor computed
> distinct scales for x and y by taking the ratio between effective
> resolution in each direction and a standard 96 dpi resolution.  Since
> this ratio is then truncated to an integer (the floor) it seems to me
> that there is no sensible possibility that these two numbers
> diverge. Moreover, modern toolkits report one number as scale factor
> and we need a common interface here. For those reasons I'm arbitrarily
> picking the horizontal scale factor as THE scale factor.

I'm not sure about this.  Admittedly, I'm not an expert on screen
scales on GUI systems, but why do we need to drop the two scale
factors as part of this change, which is basically just refactoring?
I'd suggest to keep the 2 scale factors for now.  We can always
replace 2 with one later.

> +static double
> +ns_get_scale_factor (struct frame *f)
> +{
> +  return 1;  // TODO do we need to do something else here?

Please use C-style comments, /* Like this.  */, not C++ style.

Thanks.