bug#32210: Support medium weighted fonts

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: carlosjosepita@gmail.com,  32210@debbugs.gnu.org
> Date: Mon, 18 Nov 2019 18:17:27 +0100
> 
> So we have a whole a whole bunch of bugs here, apparently.

I'm not surprised.  Just look at the different conversions of weight
symbols into numbers, and you will see the mess.  For example,
font_parse_fcname obviously supports only a subset of weight values,
so it's enough to convert a value back and forth to get a mismatch, at
least in principle.