[Fontconfig] Xft with multiple DPIs/multihead doesn't support different DPI

[Ben Stern]

In my fontconfig/conf.d, I can specify fontconfig XML which states a given
DPI, but I can't specify this per-head.

I'm running multihead on monitors with different resolutions, and my choices
are huge fonts on the lower-resolution monitor and normal sizes on the
higher resolution monitor, or unreadably small fonts on this hi-res monitor
and normal size on the low-res monitor.

xdpyinfo | grep inch correctly tells me
    on :0.0: 157x157 dots per inch
    on :0.1: 94x94 dots per inch

xrdb -query | grep dpi correctly tells me
    on :0.0 XFt.dpi: 157
    on :0.1 XFt.dpi: 94

farad:~/.config/fontconfig/conf.d$ grep -i -A2 dpi *
    19-DisplayProperties.conf: <edit name="dpi" mode="assign" binding="same">
    19-DisplayProperties.conf- <double>157.0</double>
    19-DisplayProperties.conf- </edit>

Note that without the 19-DisplayProperties.conf setting the DPI to 157
explicitly, the menus and other fontconfig-rendered text on my hi-res
monitor appears to be defaulting to the historic 96 DPI setting, and are
therefore tiny, because they're approximately 60% of the size they ought to
be.

How can I tell fontconfig to use the correct DPI on a given head?

(I tried to open this as a bug and was directed here instead.)

Thanks,
Ben
-- 
Ben Stern
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