This is a combined cherry-pick of three consecutive commits from main:
b088d77d580b8f7 GTK: hard-code some last-ditch fallback fonts.
7f4cccde2ae53c0 GTK: fixes to the previous font fallback patch.
6155365076c47a8 GTK: switch the default to client-side fonts.
The combined effect is that now PuTTY's built-in default font is
client-side rather than server-side (advantaging Wayland and
disadvantaging legacy GTK1 builds, which seems like a sensible
tradeoff these days), and also, if the configured main font can't be
found, we'll try falling back to either the client- or server-side
default (whichever is available) before giving up completely and
whinging on standard error.