Merges the fields that are set in desc_to_merge into the fields in this.
If 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.
If desc_to_merge is null, this function performs nothing.
| this |
a `PangoFontDescription` |
| desc_to_merge |
the `PangoFontDescription` to merge from, or null |
| replace_existing |
if true, replace fields in this with the corresponding values
from |