Need to set sf->save_to_dir from value of s2d, or else saving .sfdir to .sfd will save as directory.

Test case:

Open("test1.sfdir")  # test1.sfdir is any font in directory form
Save("test2.sfd")   # test2.sfd should be same font as a regular file

Broken as of 2bf376ed03cafc08b773a5870218982b4909b6e8, worked correctly prior to that.