Merges the fields that are set in
desc_to_merge into the fields in this, without copying
This is like [method@Pango.FontDescription.merge], but only a shallow copy is made of the family name and other allocated fields.
this can only be used until
desc_to_merge is modified or freed. This is meant to be used when the
merged font description is only needed temporarily.
the `PangoFontDescription` to merge from
if true, replace fields in this with the corresponding values from