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.