Merges the fields that are set in
desc_to_merge into the fields in this.
replace_existing is false, only fields in this that are not
already set are affected. If true, then fields that are already set will be replaced as well.
desc_to_merge is null, this function performs nothing.
the `PangoFontDescription` to merge from, or null
if true, replace fields in this with the corresponding values from