pango-font-description-merge (desc desc-to-merge replace-existing)

desc -- a pango-font-description instance
desc-to-merge -- a pango-font-description instance to merge from, or nil
replace-existing -- if true, replace fields in desc with the corresponding values from desc-to-merge, even if they are already exist

Merges the fields that are set in desc-to-merge into the fields in desc. If replace-existing is false, only fields in desc that are not already set are affected. If true, then fields that are already set will be replaced as well.

If desc-to-merge is nil, this function performs nothing.