bug#37752: Scaling factor inconsistencies between X with and without GTK

[Carlos Pita <carlosjosepita@gmail.com>]

In many places in xterm.c you have code like:

#ifdef USE_GTK
  scale = xg_get_scale (f);
#endif

Sometimes scale is only used in GTK specific code and that's ok.

But sometimes it's used in code that's not particular to GTK, for example:

int scale = 1;
#ifdef USE_GTK
  scale = xg_get_scale (f);
#endif
FRAME_CR_SURFACE (f) =
    cairo_image_surface_create (CAIRO_FORMAT_ARGB32,
      scale * FRAME_PIXEL_WIDTH (f),
      scale * FRAME_PIXEL_HEIGHT (f));

Besides, there is x_get_scale_factor, also defined in xterm.c, which
computes the scaling factor in a GTK-independent way, and also
discriminates between x and y scaling factors. It's only used by
x_draw_underwave.

I would like to abstract both cases inside x_get_scale_factor, that
would then use xg_get_scale if in GTK or compute just one scaling
factor (for x or for y) otherwise. Notice that given that the scaling
factor is the ratio of the effective resolution to a base resolution
of 96 dpi *truncated down* to the nearest integer, AFAICS it's almost
impossible to get different scaling factors for x and for y.

What do you think?

Best regards
--
Carlos