Enable subpixel positioning for text only on high scaling factors

Enable subpixel positioning for text only on high scaling factors

With our current font rendering stack, subpixel positioning simply does
not look good on non-HiDPI displays compared to font hinting.

While we have a setting as a way to restore font hinting, it's fairly
clunky to use with sandboxed applications, since it requires injecting a
settings.ini file in every application's configuration directory, or
adding the user's own configuration directory into the sandbox.

As a workaround, we can check the scaling factor used by GTK, and only
enable subpixel positioning if the factor is greater than one.