Set USE_MY_METRICS similar to how ufo2ft does it

[rsheeter]

Builds on #266. Many Oswald glyphs now set USE_MY_METRICS and produce identical results to fontmake. Improves results for #250 (comment), 3.

[anthrotype]

quoting from my previous self in googlefonts/ufo2ft#47

for fonts which are unhinted, or which are only hinted on the Y direction, this flag is innocuous, and doesn't make any difference whether it's set or not.

now I wonder if fontmake-rs should even bother with setting this flag, given that its outputs are unlikely to be hinted; and if they do get hinted, it's likely the responsibility of the hinting editor to set it...
only argument for adding this is to match fontmake-py's output verbatim, but from a functionality POV the (unhinted) font is the same with or without this.

[anthrotype]

FYI I just added --no-auto-use-my-metrics option to fontmake (not released yet, currently only in main) which should help with comparisons. I would suggest that we don't bother implementing this in fontmake-rs given it doesn't affect rendering for unhinted fonts and hinting editors can set this if they do hint metrics (which is even rarer nowadays).

[rsheeter]

Per IM discussion I still want to keep the ability to match fontmake default behavior here.