I need this in order to move forward #37689.

Besides I did some related changes proposed in #37752, which I'm no
closing in favor of this one.

Here is the commit message:

    * src/dispextern.h: add get_scale_factor API.
    * src/xterm.c:
      - Unify usages of xg_get_scale and x_get_scale_factor into a
        consolidated x_get_scale_factor that is exported by the rif.
      - Simplify scale inferring logic (see note below).
    * src/w32term.c:
      - Simplify w32_get_scale_factor in a similar way (see note).
      - Add it to the rif.
    * src/nsterm.m:
      - Just add NULL to the rif since there are no uses of any scale
        factor here. Clients are expected to interpret a missing API
        as meaning scale factor = 1.

    Note: 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.

Best regards
--
Carlos