Forms: Fix crash in forms with their own DR

When adding potentially missing fonts to the PDF file because we have written a letter like ħ that is not available on the form font, if the form has their own DR, we also need to add the font to that DR Font dict and not only to the global Form one (in theory we would only have to add it to the particular form one and not to the global, but it's easier this way and it's not like we're adding the data twice, we're just adding the Ref to two dicts)