fix: render U+FFFD replacement character instead of ? (#366)

The current behavior of rendering `?` for an unknown Unicode character
can be hard to distinguish from a typo. Use the standard Unicode
"replacement character" instead, that's what it's designed for:

https://en.wikipedia.org/wiki/Specials_(Unicode_block)

I'm making this PR as a draft because I'm not sure I did everything that
was needed to change the character set covered by the fonts. Running
that script is in its own commit. If this is proper, I'll rebase/squash
into one commit and un-draft.

Co-authored-by: Maeve Andrews <maeve@git.mail.maeveandrews.com>

lib/EpdFont/scripts/fontconvert.py

@@ -99,6 +99,9 @@ intervals = [
     # (0xFE30, 0xFE4F),
     # # CJK Compatibility Ideographs
     # (0xF900, 0xFAFF),
+    ### Specials
+    # Replacement Character
+    (0xFFFD, 0xFFFD),
 ]
 
 add_ints = []