x11-toolkits/pangomm: update to 2.46.4

Converted to meson and removed REFERENCE_PORT vestiges. PORTSCOUT
limited to the 2.46 series as 2.48 and later have an incompatible
API intended for GTK4 consumers.

Changelog: https://gitlab.gnome.org/GNOME/pangomm/-/blob/2.46.4/NEWS

Reviewed by: tcberner, arrowd
Differential Revision: https://reviews.freebsd.org/D33940