[ft] ttfautohint strong-stem-width commandline default

[Øyvind Kolås]

I am now running with --strong-stem-width='' for all invocations of
ttfautohint in my font generation pipeline. Which I think is a more
reasonable default than the current 'G'.

The vertical placement of horizontal stems seems to me to ba an
ortogonal to RGB subpixel vs Grayscale horizontal sub-pixel
rasterization.

The two strings I think might be most useful are '' and 'gGD', is
there a reason for not just making this a boolean toggle?

I didn't at first realize what the following in the --help implied.

  -w, --strong-stem-width=S  use strong stem width routine for modes S,
                             where S is a string of up to three letters
                             with possible values `g' for grayscale,
                             `G' for GDI ClearType, and `D' for
                             DirectWrite ClearType (default: G)

It took me some rounds of randomly juggling switches and knobs
throughout my font rendering stacks - trying to figure out why the
rendered shapes in chromium differed so much from gtk+, opera, firefox
and epiphany, turns out that the hints were different due to it
requesting/doing rendering with "grayscale vertical hints".

/pippin