If FcNameParse returns NULL FcDefaultSubstitute segfaults
We log and throw an error here because the signal emitter swallows all
exceptions so if that exception was thrown, the bar would stop rendering
without any indication as to why.
Fixes#1435
This tries to mimic the old renderer's behavior as closely as possible.
In the absence of any information, DPI is assumed to be 96x96. DPI can
be configured on a per-bar basis using the configuration key "dpi".
To use the DPI configuration from Xresources (if built with support),
one can specify the following in the bar config:
dpi = ${xrdb:Xft.dpi:96}
