kernFeatureWriter: trim value should probably be dependent on UPM

[frankrolf]

Playing around with some custom-UPM fonts, I realize:

The automatic trim value of 3 units is “small” (almost imperceptible) in the kerning of a 1000-UPM font, but it’s equivalent to 15 units in a 200-UPM font (quite noticeable). For a 2000-UPM font, it’d be 1.5 equivalent units (definitely imperceptible).

On next iteration, let’s make the threshold UPM / 1000 * 3 instead of just 3.